Detailed Magnetic Survey Research Articles

Buried Mesozoic volcanic–plutonic fronts of the north-western Pacific island arcs and their tectonic implications

Detailed magnetic surveys around the islands of Japan have revealed the presence of Mesozoic volcanic and plutonic belts at depth beneath the continental shelf. These belts, which can be correlated with those on the mainland of eastern Asia, provide a record of the migration of island arcs since the opening of marginal seas during the Tertiary.

Certain Aspects of Magnetic Studies of Charnockites at Waltair, India—Field Investigations

A detailed magnetic survey conducted in the Waltair area indicated that the charnockite rocks are more magnetic than the country rocks, khondalites and leptynites. The magnetic data suggest the possible occurrence of some unexposed charnockites. Some inferences regarding the depth of occurrence, width, and the direction of magnetization of the causative bodies have been drawn.

Detailed magnetic surveys in the Northeast Atlantic and Labrador Sea

A complex history of ocean floor spreading in the northeast Atlantic is revealed by detailed marine grid surveys of the magnetic field between Reykjanes ridge and the British Isles and between southern Greenland and Labrador. The spreading that separated Greenland from Rockall bank commenced about 60 m.y. B.P. (anomaly 24 time) at about 1.7 cm/yr, then declined to a low of about 0.7 cm/yr at about 30 m.y. B.P. A triple-ridge junction existed over this interval, with the mid-Labrador Sea ridge (Ran ridge) spreading at about 0.6 cm/yr until extinction during the middle Tertiary. Greenland then began to move as a part of the Americas plate, and the resultant change of spreading direction between Greenland and Europe apparently induced the creation of transform faults spaced 100 km apart. Spreading rates were high again (1.1 cm/yr) by 10 m.y. B.P.; the acceleration phase between 30 and 10 m.y. B.P. is poorly established. Transform faults began to disappear during this phase by gradual rotation of spreading axes, beginning about 40 m.y. B.P. in the north and gradually extending to the south. By 10 m.y. B.P., the Reykjanes ridge had become a continuous oblique ridge. Transform fault extinction progressed from northeast to southeast at about 2 cm/yr. Transform faults of short offset may thus form and disappear repeatedly during the life of an ocean basin; the present ridge crest does not necessarily mirror the original break in detail. The reorientation of spreading axes proceeded at about 1° to 4°/m.y. Before 60 m.y. B.P., an older North Atlantic basin, now deeply buried by sediment, apparently extended into the Labrador basin. South of Rockall bank, the 60 m,y. B.P. rejuvenation of spreading is marked by a distinctive ridge, and the younger basement is systematically elevated over the pre-60 m.y. B.P. basement. Magnetic anomalies and topography reveal reorientation of spreading axes and the creation of new transform faults during the period 60–55 m.y. B.P. to the south of the Charlie Gibbs fracture zone. The total offset of the Charlie Gibbs double fracture zone has not changed significantly since 60 m.y. B.P., but the evolution of a short spreading center between the two fractures is not well known. Magnetic anomalies are truncated by the Charlie Gibbs fracture zone; azimuth changes in fracture trend are suggested at 60 and, less certainly, at 38 m.y. B.P. The peculiar spreading history of the northeast Atlantic appears to have been controlled by variable discharge from a mantle convection plume now under Iceland. Asthenosphere flowing outward from the plume could have provided a large component of viscous stress on the plate bottoms in this region. A discharge peak at about 60 m.y. B.P. split Greenland from Europe and poured out large volumes of basalt onto and into the continental edges and the sea floor south of Rockall bank. Discharge declined subsequently, perhaps in concert with that of other plumes, slowing the plate motion. The Labrador and Norwegian ridges became extinct about 30 or 40 m.y. B.P. A discharge renaissance began afterward, more recent events leaving the Iceland plume about 17 and 7 m.y. B.P. to create the steplike topographic profile of the Reykjanes ridge. The oblique shape of the present Reykjanes ridge may derive from longitudinal stress exerted on the crestal area by southward-flowing asthenosphere; alternatively, the flow weakened the crestal area preferentially, steering the cracking toward a new configuration. Rockall and Seabight troughs are proposed to date from the time of the Bermuda discontinuity in the central Atlantic, variously dated at 135 or 110 m.y. B.P. It is suggested that Hatton bank is pre-Grenville in age, Rockall bank is Grenville, and Rockall trough developed along the Caledonian front. Crustal subsidence versus age in the area of the Reykjanes ridge is compared with the published subsidence history for the more typical (less elevated) North Pacific. Compared to the North Pacific standard, crust in the Reykjanes ridge area is anomalously high by values that depend on crustal age and distance from the Iceland plume. Although crust of all ages is higher than coeval Pacific crust, the anomalies are greatest for crust 0 to 20 m.y. in age (+1.3 to +1.8 km at 57°–61°N and +0.1 to +0.6 km at 47°–51°N) and for crust 60 to 40 m.y. in age (+1.5 to +1.8 km at 57°–61°N and +0.5 to +1.2 km at 47°–51°N). Because these times were also apparently characterized by higher plume discharge, it is proposed that a more buoyant, basalt-rich lithosphere is produced during times of intensive plume convection in the mantle.

Detailed Magnetic Surveys in the Star Range, Beaver County, Utah

Vertical-field ground magnetic data were obtained in four areas of the Star Range, and interpreted in terms of causative geologic structures. Eight magnetic profiles were taken over the Milford Flat intrusive body, and provided the basis for interpreting the intrusive structure under the alluvium. Gridded magnetic data were obtained in the vicinity of the Wild Bill mine. No extensive zones of contact mineralization are apparent in this data; an anomaly in the south-central portion of the grid is attributed to an upward protrusion of the local intrusive body. Along the southeastern contact of the Shauntie intrusive body, magnetic data show four areas of appreciable magnetite mineralization ranging from eight to twenty-five feet thick. Data collected south of the exposed Shauntie intrusive show an anomaly of uncertain origin that probably reflects the buried southeastern contact zone.

Tectonic Evolution of the Southern Gulf of Mexico

A detailed magnetic survey in the southern Gulf of Mexico shows subdued irregular magnetic anomalies that are similar in wave length to those attributed to sea-floor spreading on present-day oceanic rises. The small amplitude of these anomalies, about 75γ, would be compatible with an oceanic basalt source at a depth of 10 km, and previous seismic refraction studies in this area have shown that layer 2 of the oceanic crust (presumed to be basalt) does indeed lie at approximately that depth. Palynomorphs in Deep Sea Drilling Project samples of cap-rock material from the Sigsbee Knolls have shown the associated salt to be Jurassic. A crustal model, based on seismic refraction evidence and on new gravity data, suggests that, whereas the salt-dome belt of the southern gulf is underlain by a thick layer with a density of 2.2 g per cm 3 (presumed to be halite rock), adjacent deeper areas of the basin seem to lack this low density layer. In northeastern Mexico, Triassic red beds fill grabens that are correlative with the Newark Group of the Atlantic Coast and suggest that the Gulf of Mexico originated at the time of the initial rifting of the North Atlantic. When the gulf was about half-opened during the Jurassic, oceanic circulation was restricted; and thick deep-basin evaporite deposits, analogous to those found in the Mediterranean Sea by the Deep Sea Drilling Project, were laid down. Further opening established normal salinity and led to the development of salt-free areas of oceanic crust that separated the Sigsbee Escarpment (together with adjacent ridges of offshore Mexico) from the Sigsbee Knolls and salt domes of Cuba. The subsequent structural evolution of the Gulf of Mexico basin is believed to have been mainly a result of interaction between it and tectonic plates of the Pacific area. After opening of the gulf, subduction began along the Cuban arc, where Atlantic rifting had created a nearly uninterrupted tract of oceanic crust extending from the Pacific between Yucatan and Colombia. Then, folding and gravity sliding associated with Laramide deformation on the west side of the gulf led to salt anticlines that underlie the ridges offshore from Mexico. Except for continued diapirism and subsidence associated with sediment loading, present tectonic activity is confined to the southwestern corner of the gulf, where volcanism and intermediate-focus earthquakes are a distant manifestation of subduction along the Middle America Trench on the Pacific side of Mexico.

A detailed heat flow, topographic, and magnetic survey across the Galapagos Spreading Center at 86°W

The Galapagos spreading center connects the triple junction at the equator in the eastern Pacific to the Panama fracture zone. The spreading center passes north of the Galapagos Islands and into the Panama basin between the Cocos and Carnegie ridges. Between 88°W and 85°30′W symmetric high-amplitude magnetic anomalies out to anomaly 2′ (3 m.y.), giving a uniform rate of 3.25 cm/yr (32.5 mm/yr), have been observed over the crest of the spreading center. A large short-wavelength negative within the central anomaly and associated with the sharp bathymetric peak at the axis is used to determine the magnetization within the central anomaly. A uniform magnetization of 0.025 emu/cm3 (25 A/m) was used for the central block to match the anomaly associated with the bathymetric peak. An intensity of 0.0125 emu/cm3 (12.5 A/m) elsewhere and a block thickness of 0.5 km give the best match to the rest of the magnetic profiles close to 86°W. To the south of the spreading center anomaly 2′ abuts the Carnegie ridge, whereas to the north there is a change in character of the anomalies at anomaly 2′. Then, further to the north, anomalies 3 (4 m.y. B.P.) through the beginning of 5 (9 m.y. B.P.) terminate at the base of the Cocos ridge. These differences in age are evidence that the Carnegie and Cocos ridges may be tectonically unrelated. Fifty-seven heat flow measurements are presented. Thirty-eight of these were taken during a detailed survey on either side of the Galapagos spreading center. In the survey area the heat flow is high (4.08 ± 2.40 μcal/cm2 sec (171 ± 101 mw/m2) but variable. Temperature versus depth profiles in the bottom water taken during the heat flow stations show a 0.04°C increase in potential temperature over the spreading center. Hydrothermal circulation is invoked to account for the scatter in the heat flow data. In the simple plate model, vertical intrusion at a uniform temperature is assumed at the ridge crest. We show that loss of heat by hydrothermal circulation can explain the difference between theoretical and observed heat flow and that this loss of heat can be hidden in the thermal anomaly in the bottom water over the spreading center. This process not only removes the singularity in the heat flow in the original model but also flattens the temperatures near the origin. A much-improved fit to the uniform increase of depth with age of the oceanic crust observed in the North Pacific results.

Asymmetric spreading across the Juan de Fuca and Gorda rises as obtained from a detailed magnetic survey

Marine magnetic data collected along 37 east-west lines off the Washington and Oregon coast between 42°N and 48°N westward to 144°W have been interpreted. Our interpretation shows that decrease in the offsets of the anomalies across the Surveyor fracture zone and the presence of undisturbed north-south lineations east of it are the result of continuous asymmetric spreading along the Surveyor fracture zone. The survey has delineated the extension of the Blanco fracture zone north-westward to about 133°W. Movement along the Blanco fracture zone was initiated about 15 million years ago. The undisturbed north-south trend of the magnetic anomalies between latitudes 42°N and 48°N and longitudes 133°W and 136°W is interpreted as the interval (22 to 15 my) during which the Juan de Fuca and Gorda rises were one continuous structure. West of 137° the Surveyor, Sedna and three minor fracture zones are mapped.

Związek właściwości magnetycznych ze składem skał suwalskiego intruzywu anortrozytowego

RELATION BETWEEN MAGNETIC PROPERTIES AND ROOK COMPOSITION OF THE SUWAŁKI ANORTHOSITE INTRUSIVE Summary Detailed magnetic survey has been made for the rocks of the Suwalki anorthosite massif. The research comprised the measurements of magnetic susceptibility, magnitude and direction of natural magnetic remanence in rock samples, as well as the research of magnetic stability. The measurements of the magnetic susceptibility of drill cores, and the measurements of magnetic susceptibility and natural magnetic remanence of rock samples taken from drill cores allowed the magnetic differentiation of the rocks in study to be determined (Fig. 1), and the dependence of magnetic susceptibility upon the ore mineral concentration in these rocks to be established (Fig. 4). Moreover, they yielded quantitative data for the interpretation of the magnetic anomalies. The reverse direction of the natural magnetic remanence of the ilmenite-magnetite ores explains the existence of a small magnetic anomaly on the surface. The research of the magnetic stability, the thermomagnetic analysis, and the microscope examinations gave detailed data as concerns magnetic properties of the rocks under examination, as well as allowed the relation of the magnetic properties of anorthosites, norites and ilmenite-magnetite ores with their mineral composition to be determined. The research is a basis to ascertain that apart from a low percentage of ore minerals, observed under the microscope as single grains, the type anorthosite of the Suwalki anorthosite massif reveals a considerable thermal magnetic remanence characterized by the positive inclination found to occur between 21° and 40°, the magnitude of wihich depends upon the amount of thin blades of ore minerals that are found in the plagioclase crystals (Fig. 2, Table 1). There appears also another kind of anorthosites, distinguishing itself by the greater amount of ore minerals. The magnetic properties of these anorthosites are approximate to some kinds of norites (Fig. 3, Fig. 5). Magnetite or titaniferous magnetite, characterized by Curie's point amounting to 450°­–500°C, is in the norites the main ferromagnetic component deciding upon their magnetic properties. There occur here also two phases of the haematite – ilmenite series with the Curie's point 60°–130°C and approximately 300°C (Fig. 5). Due to magnetic stability, composition and amount of ore minerals, the ilmenite-magnetite ores have been subdivided into two kinds. The magnetic properties of the first kind are determined by the occurrence of only one ferromagnetic component, i.e. magnetite, whereas the second kind of ores reveals, in addition to the magnetite, also another main ferromagnetic component, i.e. ilmenite (Figs. 6 and 7). The ilmenite-magnetite ores, particularly, however, ores of the first kind, are as a rule magnetized contrary to the present-day Earth's magnetic field. Examples of reverse magnetization of rocks are presented, and this is a basis to suggest that the reverse magnetization of the ilmenite-magnetite ores here examined can be due to the self-reversal of the magnetic remanence. Normal 0 21 false false false PL X-NONE X-NONE /* Style Definitions */ table.MsoNormalTable {mso-style-name:Standardowy; mso-tstyle-rowband-size:0; mso-tstyle-colband-size:0; mso-style-noshow:yes; mso-style-priority:99; mso-style-qformat:yes; mso-style-parent:; mso-padding-alt:0cm 5.4pt 0cm 5.4pt; mso-para-margin-top:0cm; mso-para-margin-right:0cm; mso-para-margin-bottom:10.0pt; mso-para-margin-left:0cm; line-height:115%; mso-pagination:widow-orphan; font-size:11.0pt; font-family:Calibri,sans-serif; mso-ascii-font-family:Calibri; mso-ascii-theme-font:minor-latin; mso-fareast-font-family:Times New Roman; mso-fareast-theme-font:minor-fareast; mso-hansi-font-family:Calibri; mso-hansi-theme-font:minor-latin;}

Geophysical Investigations East of the Magdalen Islands, Southern Gulf of St. Lawrence

Detailed gravity and magnetic surveys confirm seismic refraction evidence of a deep (8–10 km) Carboniferous infilled sedimentary basin (the Magdalen basin) between the Magdalen Islands and Newfoundland. The basin margins are associated with gravity and magnetic 'highs' indicating shallow depths (1–3 km) to pre-Carboniferous basement rocks east of the Magdalen Islands and between Cape Breton Island and Newfoundland. The central Magdalen basin is associated with a large amplitude gravity 'low' (50–60 mgal). This can be partly explained by a 2–4 km thickness of low density Late Mississippian sediments overlain by 1–2 km of Upper Pennsylvanian or younger sediments. The western Magdalen basin is associated with large amplitude (20–40 mgal) circular and elongate gravity 'lows'. Detailed interpretations suggest they are caused by low density evaporite structures. The circular 'lows' are interpreted as salt diapirs. The elongate 'lows' are interpreted as structures in the crest of Mississippian/Pennsylvanian anticlines similar to occurrences of Windsor Group evaporites in northern Nova Scotia. The western Magdalen basin may represent one of the largest areas of salt accumulation in Eastern Canada.

A detailed magnetic study of Cobb Seamount

Results from a detailed magnetic survey and paleomagnetic measurements on oriented rock samples from the summit of Cobb Seamount indicate that the Seamount is complexly magnetized, recording at least one field reversal. The remanent magnetization probably resides in single-domain titanomagnetite (10 mole % ulvo¨spinel in solid solution with magnetite) which is unlike that found in dredged basalts. The simplest explanation for this difference is that the Seamount's pinnacle formed subaerially.

Sea-floor spreading in the southwest Indian Ocean

Results are presented from a detailed bathymetric and magnetic survey of a selected area of the southwest Indian Ocean ridge near 44°S, 38°E. The survey area is traversed by two fault zones, believed to be active transform faults. Outside these fault zones a 1000–2000 gamma peak to peak central magnetic anomaly trends roughly east-west. Some findings (deep troughs, fault scarps, and apparent offset of magnetic anomalies) are typical of the intersection of a fracture zone with a section of oceanic ridge. Two unusual features of this particular intersection are the width of the zone of fault affected magnetics and a topographic high that is subparallel to the transform faults. The surveyed branch of ridge is currently spreading in a N 20°E direction at about 1.0 cm per year. This agrees closely with an earlier estimate by Morgan and Johnson.

Mesozoic magnetic anomalies, sea-floor spreading, and geomagnetic reversals in the southwestern North Atlantic

A detailed shipborne magnetic survey in the western North Atlantic consisted of forty east-west lines, spaced 20 miles apart, extending between 23°N and 34°N and from 75°W to 60°W. Several magnetic provinces are evident. From west to east, there is the magnetically smooth zone previously described, a 100-km-wide rough-smooth transition, then a 300-km-wide band of correctable sea-floor spreading anomalies (the USNS Keathley sequence), whose eastern boundary (called the Bermuda discontinuity) is characterized by anomalies of rather high amplitude, and finally a region in which no certain anomaly correlations could be found. A model is derived for the Keathley anomalies, which, based on three Joides deep drill holes, are supposed to have been generated between approximately 155 and 135 m.y. B.P. at a rate of at least 2.4 cm/yr. This value, based on Joides drilling results, is the average spreading rate between 155 and 120 m.y. B.P. as measured parallel to transform faults discovered by an unpublished airborne survey. Closely spaced fracture zones between 120 and 135 m.y. B.P. suggest slow spreading during that time. This implies a rate exceeding 2.4 cm/yr for the Keathley anomalies between about 34°N, 63°W and 36°N, 67°W. An assumed spreading rate of 3 cm/yr for the Keathley sequence near 30°N leads to a reversal frequency of 3 per million years, approximately the upper Tertiary value. Possible left-handed transform fault offsets occur in the Keathley sequence at about 23°N and 28°N. Certain resolution of smaller offsets must await more detailed surveys. As none of the observed offsets approach the 180-km offset of the present ridge at 24°N, it is concluded that the present ridge trace is not an exact image of the early break. The paleospreading pole for the Keathley sequence is found from the rate of northward narrowing of the anomalies and from possible transform faults. The rate of northward increase of anomaly amplitude is compared with models based on various magnetic poles along the North American polar wandering curve. The agreement seems best for a pole between the Triassic and the Jurassic poles, the latter being taken near the present geographic pole, but scatter in the data is too great to be certain. If the magnetized layer is 0.5 km thick, its magnetization intensity is about ±0.005 km. Using linear cross correlation, predominate northeast, northwest, and north trends are found in the magnetic fabric of the uncorrelatable region. Comparison with bathymetric studies suggests that the first two are ‘ridge’ and ‘fracture zone’ trends, respectively. We speculate that the rift between South America and Africa developed about 135 m.y. B.P., causing spreading rates to fall and directions to change in the North Atlantic, thus producing the Bermuda discontinuity, a band of high, rough basement and high-amplitude magnetic anomalies. The high basement ridge formed 135 m.y. B.P. apparently affected the sedimentation pattern in the western North Atlantic throughout the Cretaceous and most of the Tertiary. The uncorrelatable region is characterized by closely spaced fracture zones and perhaps slow spreading. A reconstruction of the North Atlantic and surrounding areas is made to the time of the Bermuda discontinuity.

Geophysical exploration for buried river channels in the Gulgong Goldfields N.S.W.

A detailed magnetic and gravity survey was done on portion of the Gulgong Goldfields, New South Wales. The geophysical survey was undertaken to study the geology in an area blanketed by a thick alluvial cover and in particular to investigate a buried, Tertiary river system. This system is known to be present and was once the object of active gold mining operation. The magnetic survey revealed a complex anomaly pattern containing both normal and reversed anomalies. These features were difficult to interpret due to the paucity of geological information and did not appear to be related to the channel structures. The gravity survey was successful in distinguishing the generally broad bedrock depressions associated with the gold-bearing channels. Owing to the presence of a complex regional feature, qualitative interpretation was simplified by applying a number of second derivative operators to the gravity data. Quantitative interpretation wer known ground suggested that lateral density variations within the bedrock or alluvium did not significantly contribute to the measured anomalies and that a density contrast of 0.5 gm/cc exiats between the bedrock and alluvium.

Seismic, gravity and magnetic studies, Broadlands geothermal field, New Zealand

Information about structural features within the Broadlands field was sought using detailed seismic, gravity and magnetic surveys. Seismic measurements using the refraction method show the presence of several dome-shaped structures (rhyolite domes) but the greywacke basement could not be outlined. The gravity and magnetic surveys have been of little use in mapping structural features within the volcanic overburden. A residual gravity anomaly of up to +10 mgal covers most of the Broadlands field and is mainly caused by densification of the rocks within the field brought about by hydrothermal alteration of various rocks. This anomaly makes it difficult to determine the basement relief beneath the Broadlands field. Magnetic measurements give no information about the extent of the thermal area at depth. Volcanic rocks from inside the field were found to be non-magnetic.

Archaeological surveying utilizing a high-sensitivity difference magnetometer

Two pairs of alkali vapor magnetometers, one cesium, the other rubidium, have been employed in a difference mode together with a digital readout unit, for a detailed magnetic survey of an archaeological site in southern Italy. Fine grid intervals together with a system sensitivity approaching 0.05 y have enabled quantitative interpretation of small anomalies subsequently verified by drilling. High speed of operation, cancellation of time-varying fields, and direct digital readout allow generation of magnetic contour maps in the field. These features are well suited to the peculiar problems of archaeological magnetometry, and indeed to other geophysical applications.

Some Geophysical Observations on the Great Glen Fault

EARLIER geologists1–3 have considered the Great Glen fault as a normal or dip-slip fault, while Kennedy4 interpreted it as a strike-slip fault, basing his view chiefly on a lithological similarity between the Strontian and Foyers granitic complexes, and he suggested that the former has been moved 65 miles south-west along the Great Glen fault. Shand5 has not found such characteristic features of strike-slip faults as slickensides or friction-grooves and mylonite. Collette6 thinks that the fault passes to the south-east of the Shetland Islands. Flinn7 has examined the submarine topography of the Moray Firth and has outlined a possible course of the fault. Wilson8 joins the Great Glen fault to the Cabot fault of North America. Riddihough9 has conducted a detailed magnetic survey off the north coast of Ireland and suggests the presence of two faults which may have some relation to the Caledonian fault systems of Scotland.

Magnetic Survey off the North Coast of Ireland

THE geological relationships between the Pre-Cambrian and Caledonian folding and faulting systems of Scotland and Ireland to a large extent remain unresolved because of the unknown geology beneath the North Channel. A small but detailed (0.25–0.5 mile line spacing) total field magnetic survey off the coast of Donegal shows the presence of two faults which may well be of importance in the evaluation of these relationships. A simplified map of the magnetic results is shown in Fig. 1.

GEOPHYSICAL SURVEY OF THE ARKANSAS BAUXITE REGION

Magnetic and gravimetric surveys were conducted over an area of approximately 1400 square miles in the bauxite district of central Arkansas. The primary purpose of these surveys was to discover any possible buried and hitherto unknown syenite masses favorable for the occurrence of bauxite and to determine the approximate position of the buried flanks of the known syenite masses which might offer conditions favorable for the discovery of new ore bodies. These surveys indicated that the various syenite outcrops are domes or bosses on a large batholith and that other similar domes occur on the batholith but do not outcrop. Drilling on the local geophysical anomalies proved the presence of 10 buried domes, but only 2 were found to project above the upper surface of the Midway clays, a requisite of conditions favorable for the occurrence of bauxite ore bodies. The geophysical data indicated the approximate configuration of the buried flanks of the known syenite outcrops, and the portions of these flanks that project above the Midway have now been outlined more accurately by drilling. The geophysical surveys have produced evidence permitting the elimination of a large area as unfavorable for the occurrence of bauxite. Magnetic surveys extending along the Midway‐Wilcox contact from Gurdon in Clark County on the southwest to Searcy in White County on the northeast have proved the improbability of the existence of other syenite masses similar to those found in Pulaski and Saline Counties. A detailed magnetic survey of the Magnet Cove area in Hot Spring County has proved that the syenite mass exposed in that locality is an isolated intrusion and entirely unrelated to those of Pulaski and Saline Counties. This syenite mass does not occur under conditions believed to be favorable for the occurrence of bauxite.

Origin of the Salton Volcanic Domes, Salton Sea, California

The location of the Salton volcanic domes and the alinement of the associated mud volcanoes strongly suggests their affiliation with the buried extension of the San Andreas fault. Two surface trends are evident-the northeastward locus of the volcanoes, and the northwestward line of mud geysers. The latter coincides almost exactly with a southeastward extension of the San Andreas fault from 15 miles to the northwest. A detailed magnetic survey indicates that the subsurface material is not confined to the immediate vicinity of the domes. Steep magnetic gradients observed over the flats some distance from the line of volcanic hills are interpreted as the magnetic effect of the sloping surface of the underground igneous mass. In consequence of wind-directed wave attack during the presence of Lake Cahuilla and Salton Sea the lighter pumiceous materials have been concentrated to the lee of the hills in wave-built terraces or "tails," which at the southernmost volcano have accumulated in sufficient quantities to be of commercial importance. At this same volcano a peripheral distribution of obsidian with relation to pumice leads to a unique interpretation of its structure and origin. Fumarolic carbon dioxide gas trapped in the valley sediments to the northeast of the domes has been recently exploited for the manufacture of "dry ice."

A detailed regional magnetic survey as an aid to geological interpretation. District: Mittagong-Bowral

A detailed regional magnetic survey as an aid to geological interpretation. District: Mittagong-Bowral