Granite Pebbles Research Articles

“Sugorokian” conglomerate with granitic pebbles in Oga peninsula

“Sugorokian” conglomerate with granitic pebbles in Oga peninsula

Sur les rhyolites et les granites prevarisques en Normandie et en Vendee

Abstract An account of the pre-Carboniferous rhyolites and granites of Normandy and Vendee, France. Precambrian granite masses and Precambrian and Cambrian conglomerates containing ancient granite and rhyolite pebbles occur in both regions, and indicate that the Carboniferous granites of the Armorican massif were emplaced during the Variscan orogeny in a zone of subsidence between the rigid mass of Normandy to the north and Vendee to the south.

Metasomatism and Intrusion in Jamaica

The nearest known pre-Cretaceous rocks to Jamaica are Jurassic in Western Cuba, and Carboniferous in Colombia, Venezuela, and Honduras, all about 500 miles away. The evolution of Jamaica seems to have been somewhat as follows. Andine (?) orogenesis on or near the site took place, accompanied by magmatic invasion; the rocks are nowhere seen “in situ”, but they have supplied pebbles of granite, granodiorite, porphyry, hornfels, vein stuff, and limestones, more or less foliated or marmorized, but not gneisses nor schists. There is no evidence that any of them are older than Cretaceous, but they go to form the oldest beds of the island, the massive Blue Mountain purple conglomerates whose composition varies from place to place and of which no natural base is seen. Interbedded with these conglomerates are fossiliferous shales and Rudist limestones of Upper Senonian to Maestrichtian age. Low down, however, there is in Eastern Jamaica a bed of algal and foraminiferal limestone, without Rudistae, which was partly torn up and incorporated as rolled pebbles in the conglomerate. Some of these pebbles contain Camerina dickersoni Palmer, a foraminifer of Upper Cretaceous age, so the pebbles are but little older than the conglomerate containing them. Laramide orogenesis took place and this series was invaded by granodiorite and other intrusions. The uplifted area was eroded and some of the igneous and associated Cretaceous rocks exposed to denudation. Depression occurred and Lower or Lower Middle Eocene Carbonaceous shale or Richmond Beds, a Flysch-like series of limestones, shales, and conglomerates, were deposited.

The Wadi Um Dud in the Eastern Desert of Egypt

THE paper refers to a small part of the great tableland of Eocene rocks be? tween the Nile and the Red Sea Hills, and between the latitudes of Assiut and Qena (Upper Egypt), termed by Dr. W. F. Hume* the Ma'aza Limestone Plateau. Within this area I have endeavoured to map the country between the borders of the Nile valley in about 260 55' N., 31 ? 40' E. along a curve south and east to about 260 42' N., 320 8' E., in the centre of the plateau, where my traverse closes on a point already mapped by Hume and Green on their way from Qena to Assiut in 1911. As a certain amount of new information is involved and as the map was completed too late to be of use in an atlas of Egypt now in preparation,it seems better that these notes should be published than that they should lie idle .f In committing them to print, I express my gratitude to Dr. John Ball, Director of the Desert Survey, and to Dr. W. F. Hume, Director of the Geological Survey, for their kindness, invaluable assistance, and encourage? ment; and I am indebted to Mr. G. W. Murray and to Mr. O. H. Little,J of those Surveys, for reading the paper and for making useful amendments. I wish also to thank Miss G. Caton-Thompson for a map of Qau Bay which is now completed and incorporated in my map. The work was done in 1925-26 under the auspices of the British School of Archaeology in Egypt, to investigate certain geological problems which related to the superficial deposits of the Nile valley and of the wadis draining into it. One of the questions to be settled was the course by which pebbles of granite and other rocks known to come from the igneous and metamorphic complex of the Red Sea Hills had reached a point so far distant from their source as Qau (see Map I). Those best qualified to judge?e.g. Dr. Hume?were con? fident that the material had been brought northward along the Nile valley from the neighbourhood of Dena, where two great wadis enter the valley: W. Qena and W. Hammamat-W. Manilla. There was, however, a theory that the material had been brought directly westward from the Hills across the Eocene plateau into the Nile valley at Qau. The problem was made important by the discovery at and about Qau of fossil bones of man and of certain Quaternary mammals bearing in their interstices pebbles of granite, quartz, and other rocks {Man, vol 25, 1925, p. 130). But the bones were never found in the granite pebble-gravel of Qau and there was no geological evidence of their age, as they had been collected and buried in dynastic times (XVIth Dynasty). In view of the circumstances I decided to go to the Red Sea Hills, via Wadi Qena, and to trace the granitic gravel from its source there on the latitude of Qau whithersoever it might lead me. I made my headquarters at Qena, at the National Bank, where the Agent, Mr. A. S. Abdallah, gave me assistance of the utmost value especially in view of my complete ignorance (at the outset) of Arabic. Later he handled the forwarding of supplies and mail, and to him I

Glacial Drift under the Saint Louis Loess

Glacial drift in the vicinity of St. Louis seems to have been described first by Professor A. H. Worthen, in 1866.' In 189o, Professor G. Frederick Wright2 reported glacial drift in the following localities within the city limits: (i) Near Forest Park, on the road to Ferguson, beneath 20 feet of loess was bed of gravel 2 or 3 feet thick, which contained granite and other pebbles two to three inches in diameter, some finely striated; (2) at Hyde Park similar section was seen; (3) in the vicinity of Shaw's Garden. Loess had been removed for brick making, uncovering extensively gravelly stratum which contained many granite pebbles. Striae were found on angular limestone fragments. The elevation was 150 feet above the Mississippi River. H. A. Wheeler, in paper On Glacial Drift in St. Louis, in 1895,3 reported blue glacial clay or till 12 feet thick, at West Pine Boulevard and Taylor Avenue, extending westward to Euclid, underneath io to 15 feet of loess. The diameter of the bowlders is said not to exceed one foot. Among the erratics reported were red and gray granite, diorite, dolerite, and quartz-porphyry. In 1896 James E. Todd4 described bowlder clay in St. Louis. His section may still be seen on Laclede Ave. near Sarah St. The till is 8 feet thick, a reddish brown or waxy red clay, containing granite and other foreign pebbles. The overlying loess is 16 feet thick. Frank Leverett, in his monograph on the Illinois Glacial Lobe,5 reported deposits of glacial derivation underlying the loess for few

The Possible Granitization of Acidic Lower Huronian Schists on the North Shore of Lake Superior

The Michipicoten and Pucaswa areas on the north shore of Lake Superior present the apparently extraordinary anomaly of containing a conglomerate well supplied with characteristic granitic pebbles, and of exhibiting no older granitic rock of similar structure from which these pebbles could have been drawn. There are, however, of earlier age, acid schists and gneisses of probable igneous origin, and apparently of like chemical composition, though of different structure from these granitic rocks. I would suggest the possibility of the derivation of the granitic pebbles in the conglomerate from a preexisting rock of like characteristics, of which the granite which cuts the conglomerate is the regranitized equivalent. By a regranitized rock I mean one which, originally a granite, quartz-porphyry, or other rock of similar chemical composition and origin, has by metamorphism been altered into an acid gneiss or schist, and subsequent to the metamorphism-or perhaps in part during the metamorphismrecrystallized back into a granite. From our present knowledge of the origin of granite, this recrystallization cannot definitely be said to be refusion, and regranitization is perhaps a more correct and exact term. The original granite, the source of the granitic pebbles, may have been the plutonic equivalent of the rock from which the acid gneisses and quartz-porphyry or felsite schists were derived. It is possible that the gneisses may represent the metamorphic remnant of the original granite. The Michipicoten Mining Division, which contains an area of some 5,000 square miles, is part of the District of Algoma, in Ontario, and is situated on the northeastern shore of Lake Superior, between latitudes 47° 30' and 48° 30'. The division was first set apart by the government of the province of Ontario on account of the aurifer-

Additional notes on the Old Red Sandstone of Strathnairn

The following note may be considered as an appendix to the paper to be found in the Society’s Transactions, vol iii., part ii., p. 213. By reference to the map accompanying that paper the district to which I am about to refer may easily be found. It is a small piece of moorland lying to the west of Loch Ashie, and extending in the same direction for fully a mile. This little district, which is about a mile square, is remarkably rich in archæological remains. It was with the view of revisiting these illustrative relics of the primitive inhabitants of our country that I accompanied Mr Fraser, a well-known member of the Society, to the district in November of this year (1882). We found numerous out-crops of Old Red Sandstone, one of which differed in some respects from any that we had formerly observed in Strathnairn. It is a foliated sandstone, the foldings in some instances being peculiarly interesting. This was probably the source from which the cairn builders procured some of their materials, for the ‶Grey Cairn,″ which is situated about four miles to the east of this, contains many specimens of this rock. Another section shows some remarkable false bedding. The layers of the rock are very thin, so thin that they might well be used for roofing slate. I have been told that they have been quarried and used for this purpose. One granite pebble was picked out of this thin bedded rock. Here also may be

“Stone Rivers”

THE interesting account of the mode of formation of “Stone Rivers,” given in a late number by Sir C. Wyville Thomson, recalls to my mind some apparent moraines which I observed, and somewhat similarly explained, some years ago in the Hartz Mountains. In a paper read before the Geological Society of Dublin in 1872,1 I thus speak of them: “The first thing that one notices on entering this valley the ‘Ockerthal’ from the north is that the bed of the stream is crowded with granite pebbles and boulders, which become of greater size as we proceed up the valley. The boulders are soon so large—many of them some tons in weight—and are situated so far up the slopes on each side, that the first idea is, we have here the morass of a former great glacier. I looked diligently for ice marks, but could see none; and I soon found that the causes which have the effect of scattering huge blocks of rock on the slopes and on the bed of the river are now at work and are slowly, but surely, altering the contour of the adjoining granitic mountains …. All over the sides and upon the summits of these mountains are scattered the most fantastic piles of immense boulders. Some of them are over thirty feet in height and form conspicuous objects in the landscape; others, again, are deep in the forest, away from pathways, and are not to be seen until one climbs quite up to them …. It is quite clear that the contiguous surfaces of the blocks in these piles are undergoing a slow decomposition, that the joints are becoming gradually looser, and in consequence the cohesion of the component pieces less and less. Sooner or later the upper portions must either slip off or topple over, and roll down the mountain side. And this is not mere theory, for I hear that every now and then a boulder does fall and comes crushing down the hill until quietly deposited near the bottom. It would appear that while the surrounding rock has been decomposed and has fallen down in the manner indicated, these heaps have the longer resisted. But they are yet to follow in their turn; when the atmospheric agencies have sufficiently done their work, gravity will come in and lower the whole.”

On Coal-pebbles and their Derivation

T he author commenced by referring to the discovery by the late Sir William Logan, as early as 1840, of a pebble of cannel coal in a layer of indurated clay at Penclawdd, and of coal-pebbles interstratified with the Pennant Sandstones in considerable abundance (Proc. Geol. Soc. vol. iii. p. 276). The late Sir Henry de la Beche also discovered coal-pebbles in several areas of the coal-field both in Monmouthshire and Glamorganshire (Mem. Geol. Surv. vol. i. pp. 193, 194). The author noticed the division of the coal-bearing strata of the part of the Welsh coal-field east of the Vale of Neath into two groups. The higher, consisting of the sandstones known as the Pennant Grit, with comparatively few coal-seams, varies from 3000 feet in thickness at the Vale of Neath to 900 feet at its eastern outcrop near Pontypool; the lower, consisting chiefly of argillaceous shales with numerous seams of coal and ironstone, is about 1600 feet thick to the lowest workable coal-seam. According to his observations, coal-pebbles are not found in the shales of the lower group, but they occur towards the base of the Pennant Sandstones, sometimes associated with granite pebbles. Sir W. Logan's pebble of cannel-coal was found in a layer of indurated clay overlying a seam of ordinary coal; and it was supposed to have been derived from a coal-seam about 2000 feet lower in the series. This view the author regards as untenable, as it would imply the elevation and denudation of the lower in

On the Geology of Nova Scotia *

THE author, in treating on the Laurentian Rocks of Arisaig, Nova Scotia, discovered by him in 1868, referred to the occurrence of pebbles of diorite, syenite, and granite in the conglomerates of the Lower Carboniferous area of the townships of Arisaig and Antigonish, and the highly micaceous character of their grits and sandstones. He had experienced difficulty in accounting for these appearances. It appeared singular, especially, that the mica seemed to increase in those strata in proportion as they became removed from known granites. The lithological character of the discovered Laurentian band appeared to account satisfactorily for the occurrence of those constituents of the Lower Carboniferous strata. There are some gneisses and porphyritic diorites; hornblendic rock in great variety; serpentines, black quartzite strata with veins of quartz, with abundance of crystals of mica (some would be disposed to call them granite veins); white syenite with stripes of green felspar and red syenite, both very sparingly hornblendic. South of these lies a Carboniferous area which seems to overlie the Laurentian band unconformably. This area is bounded on the south by a subtriangular band of metamorphic Arisaig, or middle and upper, Silurian rocks. This band is disposed in two anticlinal folds with an intermediate synclinal. The author designates it the Antigonish Sugar-loaf Band, so named from a prominent mountain of 710 feet elevation. The extreme breadth of this band, i.e. N. and S., is about five miles. The axes run easterly and westerly. The S. side of the Carboniferous area referred to rests unconformably on the N. side of the northern anticlinal, the strata in contact being Lower Carboniferous conglomerates. This area extends to St. George's Bay and Cape St. George on the Gulf of St. Lawrence; it is basin-shaped, and is said to contain seams of coal. This area may be called the Arisaig area. South of the Silurian area lies the Antigonish Carboniferous area. The lower part of this area consists of conglomerates, limestone, and gypsum. The conglomerate lies unconformably on the Silurian slates of the S. side of the southern anticlinal. The slates dip < 55° S. 5 E.; the Lower Carboniferous conglomerates and limestones dip < 30° S. 35 W. The observations made are at variance with two theories that have been advanced by different geologists to the effect—1st, that the Upper Silurian and Devonian formations of Nova Scotia have been thrown into a few great folds, synclinal and anticlinal; 2nd, that the Carboniferous and underlying Devonian or Upper Silurian formations acquired their present positions simultaneously, the mountains having had a thick Carboniferous saddle, which had been subsequently removed by denudation. This theory supposes that the Carboniferous areas of Nova Scotia had been once united with each other and those of other countries. The Nova Scotia areas that are now separated have always been so, the only connection ever existing having been merely geological.

XXVII. Notice of some peculiarities observed in the Gravel of Litchfield.

The red sand or gravel (for it may be called by either name) which overspreads the country in the neighbourhood of Litchfield, has presented to me some remarkable appearances; a short notice of which may perhaps without impropriety be offered to the Geological Society. The principal ingredient in this alluvial mass is quartz, in small roundish grains, mixed rather copiously with scales of silvery mica, and tinged of a brownish red colour by oxid of iron. In some places no other substances than those just enumerated make their appearance, and the soil is a loose incoherent driving sand. More generally however the grains are slightly cemented together by a little red clay, and rounded pebbles of various sizes and qualities are interspersed. I am unacquainted with the thickness of this bed, but excavations to the depth of about thirty feet have been formed in it by the side of the road to Birmingham, about two miles from. Litchfield. At this place the pebbles are so abundant as to compose a considerable proportion of the entire mass, and it is here that the appearances, which are the subject of the present notice, may be most conveniently observed. The pebbles of granite, of syenite, and of greenstone, are in a state of greater or less decomposition, but present nothing very remarkable, those of schist are usually soft and rotten, have evidently swelled since the period of their having been deposited here, in consequence of which their laminæ have parted from each other,

IV. Observations on the Mountain Cruachan in Argyleshire, with some Remarks on the surrounding Country.

The geological history of this mountain being, as far as I know, unrecorded, I shall relate the few observations which I made on it, as they are sufficiently numerous to form at least a basis for future and more accurate investigation. It is evident to any eye in approaching through the vales of Glenorchy or Glenara to the head, of Loch Awe, that the nature of the country has changed. The rugged forms and rocky faces characterizing those hills of mica slate which bound Loch Lomond, Loch Long, Loch Fyne, or Strath Fillan, have disappeared; the mountains assume a more uniform flowing line, their sides are more completely covered with herbage, and exhibit fewer denuded rocks; their summits are less serrated, and are almost the only parts which exhibit the naked rock, while at the same time they are strewed with heaps of fragments, a character from, which the hills of mica slate are almost always free. On approaching nearer to their bases, the red colour of the fragments which have fallen down from their sides, and the rounded pebbles of granite and porphyry which are met with in the beds of the torrents, give the mineralogist pretty plain intimation of the causes of this change of feature. Except in the rolled fragments however, no appearance of granite is visible as we skirt the sides of Cruachan from Dalmally to Inverawe. Every where we observe schistose rocks, which have been laid bare, sometimes by the operation of natural causes, but