Breccia Matrix Research Articles

Rb-Sr ages of clasts from within Boulder 1, Station 2, Apollo 17

Rb, Sr and87Sr/86Sr have been determined for fragments of matrix and clasts from three of the hand-specimens of Boulder 1, 72275, 72255, and 72215. Total-rock and certain plagioclase samples from a crushed norite clast (Civet Cat) define an age of 4.17±0.05AE (2σ) for the pre-Serenitatis igneous differentiation of the norite. Pyroxene and other mineral separates were affected by a later event at about 3.9±0.1AE. An unshocked clast of pigeonite basalt has a well-fitted mineral isochron of 4.01±0.04AE. Samples of the competent breccia matrix comparatively rich in small clasts of highly radiogenic microgranite define a mixing line equivalent to 4.03±0.03AE, which denotes the age of the microgranite. Other samples of the matrix dominated by small anorthosite clasts define a 4.4AE mixing-line and demonstrate that Sr isotope equilibration between plagioclase and matrix did not occur during the high-temperature event that indurated the matrix.

Light and dark soils at the apollo 16 landing site

On the basis of inert gas systematics alone, the soilsnow near the surface at the Apollo 16 landing site can be divided into three major groups: Group I (North Ray Crater Soils), Group II (Light Soils), and Group III (Dark Soils). Only five soils do not fit this scheme. The inert gas-based classification is correlated with the chemistry of the soils. Group I soils are relatively poor in K, Fe, Ti and Zn, compared to Group II and III soils. The classification is also correlated with reflectivity. Group I and II soils are generally the light soils in the landing area, while the Group III soils are the dark soils. The groups are not randomly distributed in the landing area. Group I soils occur only at stations 11 and 13 on the ejecta blanket of North Ray Crater. Group II soils occur abundantly at stations 1 and 2, and in spots on Stone Mountain. Group III soils are abundant on Stone Mountain and at station 10. We suggest here that Group I soils are principally derived from the light friable unit, one of the three units inside North Ray Crater, as described by Ulrich. We suggest that Group II soils are mainly derived from the light matrix breccia unit. Group III soils are mixtures of materials from all three units. We conclude that soils with the properties of Group III soils have been at the surface continuously for long times. However, going backwards in time, these soils probably had increasingly larger (Ar40/Ar36)t ratios. The ejecta blanket of North Ray Crater is a temporary ‘anomaly’ in the landing site. However, soils with the properties of Group I soils, but with larger (Ar40/Ar36)t ratios may turn up in Apollo 16 core tubes. The Group II soils show a record of solar wind exposure in the distant past (i.e., they have relatively large Art 40/ARt 36 ratios). From this we conclude that the regolith at Apollo 16 contains sizeable ‘pockets’ or horizons at depth which are the sources of the Group II soils. The materials in these pockets may be akin to soil 61 220.

Stratigraphy of the descartes region (Apollo 16): implications for the origin of samples

Analysis of terrain in the Apollo 16 Descartes landing region shows a series of features that form a stratigraphic sequence which dominates the history and petrogenesis at the site. An ancient 150 km diam crater centered on the Apollo 16 site is one of the earliest recognizable major structures. Nectaris ejecta was concentrated in a regional low at the base of the back slope of the Nectaris basin to form the Descartes Mountains. Subsequently, a 60 km diam crater formed in the Descartes Mountains centered about 25 km to the west of the site. This crater dominates the geology and petrogenetic history of the site. Stone and Smoky Mountains represent the degraded terraced crater walls, and the dark matrix breccias and metaclastic rocks derived from North and South Ray craters represent floor fallback breccias from this cratering event. Subsequent major cratering occurred in the region (Dollond B, etc.) prior to the Imbrium and Orientale basin-forming events but had minor effect on the site. Based on this interpretation, contributions from Imbrium at the Apollo 16 site are minor and those from Orientale negligible. The petrology of the Apollo 16 rocks supports this stratigraphic and process model of a local crater-dominated history for this region.

Galena-Bearing Pyrite Nodules in the Nelway Formation, Salmo, British Columbia

Pyrite nodules composed of radiating elongate pyrite crystals and including some galena are found in the sparry dolomite matrix of a dolomite breccia within the middle Cambrian Nelway Formation, Salmo map-area, British Columbia.Similar textures, mineralogy, host rock, and stratigraphic position for the nodules and some pyritic zinc-lead ores in northeastern Washington are taken to indicate a common lineage. Favored is an hypothesis calling for the formation of solution-collapse breccias and their filling by dolomite and sulfides precipitated from low temperature solutions. Later deformation and metamorphism erased or concealed the record of these early events in many of the ore deposits.

Geology and tectonic setting of the Mule Creek caldera, New Mexico, U.S.A.

The 10-km diameter Mule Creek caldera is the youngest felsic eruptive center in the Mogollon-Datil volcanic field of southwestern New Mexico. The caldera forms a topographic basin surrounded by a raised rim. The caldera wall is well displayed on the south and west sides of the structure where it dips 20–30 degrees toward the center of the basin. Mudflow breccia fills the caldera and is banked up against the caldera wall. Post-caldera porphyritic quartz latite domes and flows crop out along the ring-fracture zone. The caldera is superimposed upon an older volcanic complex of flow-banded rhyolite and porphyritic andesite lava. The Mule Creek caldera probably originated by explosive eruption of about 10 km3 of pumice and ash, in part preserved in the matrix of the mudflow breccia. Periods of explosive volcanism during the deposition of mudflow breccia are documented by tuffaceous beds interbedded with the breccia. A thin rhyolite ash-flow sheet originated in the caldera and overlies the mudflow breccia. The youngest felsic rocks around the caldera are (1) domes and flows of crystal-rich porphyritic quartz latite of variable mineralogy, interpreted as a defluidized magma, and (2) widespread crystal-poor, flow-banded rhyolite, dated at 18.6 m.y., which is not directly related to the caldera sequence. The Mule Creek caldera and other volcanic features farther south represent the only documented overlap of felsic volcanism with early stages of Basin-Range tectonism in the Mogollon-Datil field.

Occurrence of chromian, hercynitic spinel (“pleonaste”) in Apollo-14 samples and its petrologic implications

Many isolated grains of a reddish pleonaste-type spinel occur in fines and metabreccia samples, particularly 14 319. Electron microprobe analyses (104) of spinels and their associated phases include 58 of pleonaste which show Mg/(Mg + Fe) 0.44–0.62 and Cr/(Cr + Al) 0.017–0.134 (atomic), plus minor amounts of other ions, and differ greatly from almost all previously recorded lunar spinels; almost no spinels of intermediate composition were found. Two types of compositional zoning exist: a diffuse primary one with cores lower in Ti, and a narrow secondary one from reaction with matrix yielding rims higher in Cr, Ti, and Mn. At contacts with breccia matrix there is a narrow corona of almost pure plagioclase (An 80-An 94), free of opaque minerals and pyroxene. Two types of solid inclusions found in the pleonaste are calcic plagioclase, and tiny spherical masses of nickel-rich sulfide. Similar pleonaste occurs in crystalline rock clasts, mainly with plagioclase; one clast (A) consists only of coarse olivine, plagioclase, and pleonaste, with granulated grain boundaries suggestive of deformation. From composition and texture, this clast is one possible candidate for the mafic cumulate counterpart of the “anorthositic” crust. Another clast (B), also made solely of olivine, plagioclase and pleonaste, is itself a breccia. These data suggest a two-stage brecciation process: 1) disruption (probably pre-Imbrian) of a deep-seated pleonaste-bearing source rock like A and reconsolidation to form a breccia without addition of pyroxene, ilmenite or other minerals; and 2) disruption of this breccia to yield breccia clast B which was then incorporated into the Fra Mauro formation.

Luna 16: Relative proportions and petrologic significance of particles in the soil from Mare Fecunditatis

We have classified some 2380 lithic and vitreous particles from levels A and Γ(G) of the Luna 16 core-tube sample. The proportions of particle types in the two levels are very similar, indicating a well-mixed regolith. Light-colored, non-mare materials in the soil consist of anorthositic fragments similar to those from Apollo 11, a Ti, K, P-poor gabbro not reported from earlier missions, and their vitreous equivalents. KREEP-rich norites and their glassy counterparts are very rare. Luna 16 mare basalts have a more restricted variety of textures than do the basalts from the Apollo 11 and 12 missions and contain pyroxenes that fall into two groups on plots of TiO 2 vs. Fe/(Fe + Mn + Mg). TiO 2 is high in cores of type 1 pyroxenes, but falls sharply toward grain boundaries, probably reflecting depletion of TiO 2 in the melt by simultaneous ilmenite crystallization. Type 2 pyroxenes have lower and less variable TiO 2, but are more strongly zoned in Fe/Mg and reach higher Fe/Mg ratios than type 1 pyroxenes. Fragments with type 1 pyroxenes appear to be early crystallizing portions of a series of flows whose later stage pyroxenes are dominantly type 2. Cindery glass and breccia matrix compositions are not identical to the bulk regolith composition but contain higher normative plagioclase, indicating a greater non-mare component than is present in the bulk soil. The most likely source of non-mare material at the Luna 16 landing site is the crater Langrenus, 140 km in diameter and 280 km southeast of the landing site. It appears that a substantial part of the cindery glass found in the Luna 16 soil may have been derived from soils containing a larger component of Langrenus ejecta than is present in the Luna 16 bulk soil sample.

Pseudotachylite from the Vredefort Ring, South Africa

Pseudotachylite, a very fine grained glasslike rock, forms veins and the matrix of breccias in the core and overturned collar rocks of the Vredefort ring. The cataclastic character of coarse breccias, of which pseudotachylite forms the matrix, persists to microscopic sizes where individual mineral grains are seen to be laced by thin veins composed of crushed mineral debris. Only locally, and only in granitic rocks, is there evidence suggestive of fusion. Mixing of crushed mineral debris during injection or spallation into fractures formed the common type of pseudotachylite. Well-developed basal deformation lamellae in quartz of rock fragments in pseudotachylite and in the wall rock indicate high strain rates and transient pressures above 35 kb, and possibly above 100 kb, before or during pseudotachylite formation. Deformation of other minerals includes formation of multiple sets of planar structures in feldspars and mica, and a single set of planar structures parallel to {101 }? in amphibole. Micas and amphiboles underwent further breakdown to mattes of unoriented minute grains before incorporation in pseudotachylite. The mode of occurrence of pseudotachylite and association with shock deformation features are shared by microbreccias in carbonate rocks of cryptoexplosion structures elsewhere that are not related to magmatic activity. This supports the belief that such breccias, including Vredefort pseudotachylite, are the products of shock deformation, probably related to impact events.

Anatomy of a Meteorite Impact Crater: West Hawk Lake, Manitoba, Canada

West Hawk Lake is a near-circular, 11,700-ft -wide, deep lake located 110 mi east of Winnipeg, Canada. The lake now completely submerges an ancient meteorite impact crater. A central drillhole passes through nearly 1080ft of slump and fallback breccia into 655 ft of rupture zone bedrock below a well-defined crater base. Petrographic analysis of core from this hole, a hole near the original crater rim, and an off-center hole reveals numerous shock features which, supplementing earlier geophysical studies, demonstrate an impact origin for this structure. These results are significant: (1) The original crater was approximately 8000 ft wide; the rim has recessed by erosion; the target rock consists of steeply dipping units of metasediments and metavolcanics. (2) Microscope criteria for shock include: abundant planar features in quartz; diagnostic parallel extension fractures; extensive isotropization of tectosilicates in quartz-mica schists (analcite is a common product of recrystallization of the isotropized phases); variable decomposition of biotite and actinolite; abundant glassy fragments and matrix-binding “melt,” now generally devitrified; shock-induced twinning and granulation in pre-impact carbonates. (3) Distribution of shock features is strongly dependent on mineral composition and particle size. Unless intensely shocked, amphibole-rich rocks are usually devoid of shock evidence, even where fragments are intermixed with shocked schistose metasediments. (4) The vertical distribution within the central hole core of intensity of shock in the breccias, as plotted on a shock log, is notably erratic. (5) Autobrecciation, cataclastic fragmentation, and injection of shocked breccia into open fractures account for observed effects in the rupture zone. (6) Zoisite and secondary calcite develop extensively within the breccia lens by solution and neo-mineralization; serpentine var. antigorite forms with the breccia matrix, and in a network of veinlets in fragments and in the rupture zone; sodium-calcium zeolites form in possibly replaced melt material at the crater base. (7) Analysis of strongly shocked rock fragments from the breccias by infrared absorption, X-ray fluorescence, electron microprobe, and argon loss methods demonstrates significant changes in the crystal structure and chemistry of constituent minerals in these materials as compared with unshocked equivalents collected off-structure. West Hawk Lake is compared with the Brent structure, the best-studied model of a simple impact crater in crystalline rocks.

Solution-Collapse and Cavity Filling in the Windsor Group, Nova Scotia, Canada

Research Article| July 01, 1967 Solution-Collapse and Cavity Filling in the Windsor Group, Nova Scotia, Canada H EDWARD CLIFTON H EDWARD CLIFTON U.S. Geological Survey, Menlo Park, California Search for other works by this author on: GSW Google Scholar GSA Bulletin (1967) 78 (7): 819–832. https://doi.org/10.1130/0016-7606(1967)78[819:SACFIT]2.0.CO;2 Article history received: 10 Jan 1966 first online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation H EDWARD CLIFTON; Solution-Collapse and Cavity Filling in the Windsor Group, Nova Scotia, Canada. GSA Bulletin 1967;; 78 (7): 819–832. doi: https://doi.org/10.1130/0016-7606(1967)78[819:SACFIT]2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Limestone breccia, estimated to be as much as 100 feet thick, which disconformably overlies the basal limestone of the Mississippian Windsor Group in central Nova Scotia, has heretofore been considered sedimentary breccia. It is absent in deep borings; at depth, the basal limestone is apparently conformably overlain by interbedded limestone and anhydrite.The breccia typically is a jumbled mass of unsorted angular limestone fragments in a matrix of muddy massive limestone. Most of the fragments are similar to the basal laminated limestone of the Windsor Group although some are of massive limestone and red marl apparently derived from beds higher in the section. In a few exposures, the breccia contains relict beds; in others, thin, graded calcarenite beds appear to fill pockets in the breccia. Numerous pipes, channels, and large masses of cavity fill containing sand and scattered quartzose pebbles occur within the breccia.The breccia postdates the Late Paleozoic regional deformation that extensively folded the Windsor Group. Stratification in floored cavities is virtually horizontal and independent of the attitude of the adjacent Windsor strata. The deformation produced veins normal to bedding in limestone of the Windsor Group; many rotated fragments within the breccia contain similar veins normal to their bedding. Metamorphic fabric, absent in the breccia matrix, varies in nature and orientation from fragment to fragment. Sand grains, pebbles, and heavy minerals within the cavity fill appear to have been derived from Triassic rocks, suggesting that the breccia originated in either Triassic or post-Triassic time. The compositional dissimilarity between these pebbles and those of Pleistocene gravel indicates a pre-Pleistocene origin.It is here proposed that the breccia formed only near an erosion surface by collapse and disintegration following solution of anhydrite interbeds and part of the limestone. The cavity fill appears to be composed of surficial debris which filled solution channels within the breccia.The breccia formed long after the deposition of the Windsor Group and should not be regarded as part of the depositional sequence of the group. The name “Pembroke Formation” proposed for the breccia and cavity fill by Weeks (1948) is preempted and should be regarded as abandoned. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

A Study of the Strathcona Mine and Its Bearing on the Origin of the Nickel-Copper Ores of the Sudbury District, Ontario

A Study of the Strathcona Mine and Its Bearing on the Origin of the Nickel-Copper Ores of the Sudbury District, Ontario

Discussion

Dr D. V. A ger commented that, apart from the very interesting palaeogeographical implications of the outliers, they appeared to have some significance in connexion with the nature of the Chalk sea. The speaker inquired about the apparent purity of the chalk described and the absence of the usual shallow- to deeper-water transition of sandy and argillaceous deposits below the white chalk proper. This was particularly surprising if, as was suggested, these outliers record a transgression of the Chalk sea over Palaeozoic rocks with some relief, and also if they constitute the basal deposits of that sea tumbled down into solution-chambers. Professor W. D. G ill expressed the view that the author’s interpretation of the mode of occurrence of the chalk must be substantially correct. On the wider significance of the exposure with regard to the age of the Irish land-surface, the speaker was inclined to be more emphatically in favour of an essentially Mesozoic age for the flatter surfaces above 500 ft o.d., for there seemed to be no evidence from rock structures to support late deformations. It had to be remembered, however, that in Antrim the base of the Chalk had been considerably deformed, and that extensive Tertiary changes in summit levels must have taken place in the area of the Mourne granite. Written contribution from Professor T. N eville G eorge : The author’s interpretation of the Senonian matrix of the Ballydeanlea breccia is attractive but not wholly convincing. A ‘submarine karstic subsidence’ seems in its terms to be self-contradictory, and

The Solution Brecciation Process

The development of evaporite-solution breccias may take place by solution of the evaporite and production of large voids, followed by collapse of the nonevaporite rock. Study of solution breccias, however, in which all stages of breccia development are preserved indicate that during evaporite-solution brecciation gradual subsidence, evaporite flowage, and small-scale precipitation may occur simultaneously with gross evaporite removal; thus cavern formation and roof collapse may be relatively unimportant. As evaporite dissolves, overlying and interbedded nonevaporite strata subside and fragment; evaporite flows so that rock fragments are floating in it; a grain-supported mass gradually forms; and finally, an evaporite-free breccia. The breccias studied in cores from the Upper Devonian Leduc Formation, Alberta, Canada, consist of angular dolomite clasts in a matrix of either dolomite or anhydrite. The clasts range from single dolomite crystals and rock fragments a few tens of microns in diameter to large cobbles. In some cores the clasts float within the anhydrite matrix; in others, the anhydrite is less abundant and the clasts are in grain–to–grain contact. In anhydrite-free breccia, the dolomite matrix is composed of fine clasts equivalent to those floating in the matrix of anhydrite-bearing breccia. Many clasts are broken, but the fragments have been held in relative position in the matrix and can be identified. The basal contact of the breccia is sharp. The breccia grades upward into highly fractured in situ rock.

An intrusion breccia with associated ultrabasics from Sermersût, South-West Greenland

A detailed 1 :500 map and a description of some peculiar breccia structures are presented. The breccia was emplaced in the Precambrian basement rocks, consisting mainly of granodioritic gneisses and amphibolites. The structures of the breccia are highly irregular consisting of lots of connected dyke-shaped bodies. More distinct dykes of the breccia matrix run out into the gneisses. The breccia itself consists of a matrix of mineral fragments and numerous inclusions of rock fragments, the majority of which are of exotic origin. Associated with the breccia are some larger soapstone bodies of which smaller fragments are found in the breccia. The breccia is thought to have been emplaced by gas drilling in connection with the intrusion of the soapstones in a solid/subsolid stage. The chronological significance of the breccia is emphasized.

Pembroke Breccia: Solution--Collapse of the Lower Windsor Group (Mississippian) in Central Nova Scotia: ABSTRACT

The Pembroke Breccia disconformably overlies the basal limestone of the Mississippian Windsor group in central Nova Scotia and has previously been considered a primary breccia in the Windsor group. The breccia is estimated to be as much as 100 feet thick. It is absent in deep borings; at depth, the basal limestone is apparently conformably overlain by interbedded limestone and anhydrite. The Pembroke typically is a jumbled mass of unsorted angular limestone fragments in a matrix of muddy massive limestone. Most of the fragments are similar to the basal laminated limestone of the Windsor group, although some are of massive limestone and red marl apparently derived from beds higher in the section. Irregular pipes, channels, and masses of sandy breccia containing scattered quartzose pebbles occur within the typical breccia. In a few exposures the breccia contains relict beds; in others, thin graded calcarenite beds appear to fill pockets in the breccia. Late Paleozoic regional deformation extensively folded the Windsor group; the Pembroke Breccia is younger than this deformation. Stratification in floored cavities is virtually horizontal and is independent of the attitude of the adjacent Windsor strata. The deformation produced veins normal to bedding in the Windsor limestone; many rotated fragments within the breccia contain similar veins normal to their bedding. Metamorphic fabric, absent in the breccia matrix, varies in nature and orientation from fragment to fragment. Sand grains, pebbles, and heavy minerals within sandy parts of the breccia appear to have been derived from Triassic rocks, suggesting that the breccia originated in either Triassic or post-Triassic time. It is here proposed that the breccia formed only near an erosion surface by collapse and disintegration following solution of anhydrite interbeds and part of the limestone. The sandy breccia appears to be composed of surficial debris which filled solution channels within the typical breccia. Because the Pembroke Breccia was formed long after the deposition of the Windsor group, it should not be regarded as a stratigraphic unit of the group. The actual depositional sequence of the basal Windsor is (in ascending order): limestone, interbedded limestone and anhydrite, anhydrite, and halite. End_of_Article - Last_Page 521------------

FAULT IN PALEOZOIC ROCKS NEAR FREDERICK, MARYLAND

Diamond drill cores from an area 5 miles southwest of Frederick, Maryland, reveal significant stratigraphic and structural information. The Antietam and Tomstown (Lower Cambrian) and Grove formation (Ordovician) are involved. The Antietam has some quartzite layers but is predominantly quartzose phyllite. The lower 40 feet of Tomstown is dolomite with sericite partings. The remaining 140 feet is impure, thin-bedded dolomitic limestone. The Grove is a light-gray slightly dolomitic limestone with red staining in the matrix of breccias and along fractures. The Antietam and Tomstown dip easterly 25°–30°. Overlying the Tomstown, above a fault which also dips 25°–30° easterly, is Grove limestone which is overturned, dipping 75°–85° easterly. Along the fault are up to 5 feet of black clay gouge, 30 feet of breccia, and slickensides. The absence of the Frederick limestone and the angular relationship of the Grove bedding to the fault surface provide additional evidence of faulting. The fault, having younger rocks resting on older, could be normal, but the low angle plus overturning of the Grove limestone suggest a subsequent shear thrust. The authors postulate that this fault or a series of similar faults may explain anomalous stratigraphic relations in the Lower Paleozoic rocks along the eastern edge of Catoctin Mountain. Locally, where the Antietam and/or Tomstown formations are missing, faulting could be the cause instead of facies changes or one or more unconformities.

The Howell Structure, Lincoln County, Tennessee

A small area of highly disturbed, contorted, and brecciated strata centering at Howell, in north-central Lincoln County, Tennessee, has been mapped and studied in detail. The salient structural feature is a circular area of intensely deformed Black River and Trenton rocks, which have been uplifted approximately 100 feet above their normal positions. This circular area is composed of jumbled blocks of limestone imbedded in a matrix of shatter breccia. The major deformation is believed to have been post-Trenton and pre-Fernvale in age. Overlying the shattered strata is the Fern vale formation, the relative thickness and lithology of which point directly toward deposition in a graded crater. The Fern vale shale unit shows moderately high dips. This partial disturbance of the Fernvale possibly indicates a mild renewal of the forces which caused the major deformation. The younger Silurian and Mississippian formations are relatively undisturbed. A magnetic survey indicates a "closure" of 335 gammas, 1 3/4 miles southwest of the center of the disturbance. While no conclusive evidence has been observed to support either the cryptovolcanic or the meteoritic hypothesis of origin, the Howell structure is considered tentatively as an example of the cryptovolcanic structures as interpreted by Bucher.

The Trilobite Fauna of the Comley Breccia-Bed (Shropshire)

I. Introduction I n a report to the Dundee Meeting of the British Association (1912) upon the excavations made in the Cambrian rocks at Comley during 1911, I called attention to a Paradoxides fauna, which appears to be new to the district, and is found in the matrix of a remarkable breccia near Comley Brook. The objects of the present paper are:— (1) To figure the trilobites of this breccia-bed, and to describe those species which appear to be new; (2) To discuss the palæontological horizon in the Cambrian System which they indicate; and (3) To consider some inferences that may be drawn from the occurrence of this fauna in its present position among the rocks of Comley. II. Recapitulation of Previous Observations In order to make clear the special significance of the fossiliferous Breccia-Bed, it is necessary to recapitulate some of the observations already published. In the section of the Quarry Ridge, the Groomii Fauna is found in the matrix of a conglomeratic grit which rests upon the Lower Cambrian limestones that yield the Protolenus-Callavia Fauna. A similar conglomeratic grit is found at Robin's Tump with some of the same fossils; but in this case it rests, with visible unconformity, on bedded green sandstones, which are regarded as lower in the Cambrian sequence than the Protolenus-Callavia Limestones. The conglomeratic nature of the grit is easily recognized, for the matrix consists largely of rounded quartz-grains with a liberal sprinkling of glauconite,