large worn blocks of slaty rocks; 3ft. 6in. thick, and about 4 feet above high water at its base. Rock platforms are noticeable at about the level of spring tide high water. FIG. 1.-The Coast toward Rosemullion Head; showing Rock Platforms and Cliffs composed of Head upon Raised Beach. 6. South of the R. Helford. a. At Ligwrath, between Nare Point and Porthalla, the Head consists of brown earth with angular stones, pebbles are met with in places at its base, at about 5 feet above high water. Boulders compose the present beach. b. South of the above, traces of a raised beach consisting of beds of coarse black and brown sand, with grit, slate, igneous rock, and small quartz pebbles, in places 2 to 3 feet thick, and at base about 8 feet above high water, are visible here and there under Head of grey and brown loam with angular stones. c. De la Beche (Report, p. 431) figures part of a consolidated raised beach forming the roof of a cavern in the slates on which it rests, and supporting a Head of angular fragments, between Porthalla and the Nare Point. He also gives a sketch of the old beach at Nelly's Cove and between Rosemullion Head and Mainporth (op. cit. p. 432). d. The Rev. E. Budge (T. R. G. S. Corn. vol. vi. p. 1) mentions the occurrence of a raised beach, about 5 feet above high water, continuing for some hundreds of yards from Nelly's Cove ( mile from Porthalla), and accessible only at low water; he observed traces of the old beach on steep rock ledges now overflowed by the tide. On the north of Nare Point, 8 to 10 feet of angular debris rested on the old beach. 7. Coverack Cove. a. The low cliffs to the east of Carnsullan are about 15 feet in height, and composed of brown earth with angular and subangular stones and boulders. b. The Rev. E. Budge (op. cit.) describes the cliff-section on the north side of the Cove as-Reddish-coloured marl or rubble upon a thick bed (12 feet) of fine ferruginous sand, consolidated in places, upon large rolled pebbles arranged in regular lines and about 5 feet above high water at their base. c. The same observer says that the whole of the outer portion of the Lowlands in St. Keverne parish (a flattish tract of 60 acres in extent) is formed of very fine sand (valued for constructing moulds for brass casting), so similar to that overlying the Coverack raised beach that he considered them contemporaneous. At and near the coast-line pebbles were occasionally met with in the sand. d. Mr. Budge mentions a rampart of large diallage pebbles round a low fortress of sand upon the present beach at Coverack. e. Dr. Boase (T. R. G. S. Corn. vol. iv. p. 329) mentions the occurrence of diluvium of an ochreous colour consolidated toward its base, and containing small pebbles of quartz, compact felspar, and serpentine, resting on serpentine, near Coverack Quay. f. De la Beche (Report, p. 129) and Godwin-Austen (Q. J. G. S. vol. vii. p. 121), comment on flints occurring in the Coverack raised beach. Flints also occur in the present beach at Porthbeer Cove, south of Coverack. 8. Gunwalloe. The cliffs are capped in places by a Head of light brown loam with angular stones. The Lizard District south of a line between Porthbeer Cove and Mullion was not observed by me, nor can I find any descriptions of Pleistocene phenomena on its sea-board. The low cliffs to the south of the Loo bar are capped by about 5 feet of brown loam with angular fragments of quartz, etc., under coarse brownish blown sand. 9. Coast from Loo Pool to Marazion. a. De la Beche (Report, p. 430) figures part of a raised beach between the Loo Pool and Cove village, stained by black oxide of iron, and containing strings of the same substance, the prevalence of which in the rocks of South Cornwall is pointed out. b. Mr. Henwood (T. R. G. S. Corn. vol. v. p. 54) noticed patches of granite and slate pebbles, from the size of a nut to a foot in diameter, in Tremearne Cliff. The deposits rested on slates at 14 feet above the present beach, in one spot, and at 30 feet in another, going eastward. c. (op. cit.) "At Wheal Trewavas, where the rock is wholly composed of granite, it is covered by a thick bed of transported fragments of micaceous slate." d. On the west of Pra Sands, Mr. Henwood (op. cit.) noticed a bed of granite, elvan, and slate pebbles, at about 6 feet above the present beach, and covered by "a high bank of rubbish," the debris of the adjacent rocks. e. Between Cuddan Point and Trevean Cove, the Head consists of dark grey loam with angular (local) fragments. f. The Perran Sands are bounded by cliffs from 5 to 20 feet high, partly composed of brown loam with angular stones and blocks of greenstone. g. In a cove west of Perran Sands and south of Perranuthno; in one part― Brown earth with large and small angular stones... greenstone ... upon-brown loam with small angular quartz stones and large g. In another place Soil Brown loam with angular greenstone fragments 10ft. to 15ft. 1ft. 2ft. to 3ft. ... ... As above, fragments fewer, and, as a rule, smaller ... 6ft. to 7ft. 10ft. to 15ft. 2ft. (about). resting unevenly upon greenstone, at from 8 to 12 feet above high water. h. Toward Marazion the cliffs average 20 feet in height, and are composed of a Head of angular slate, quartz, and greenstone fragments in brown loam. 10. South of Penzance. a. Mr. Carne (T. R. G. S. Corn. vol. iii. p. 229) observed layers of pebbles and boulders from 3 to 6 feet thick, and 40 feet in length, at the junction of the slate and granite at Mousehole. Mr. Henwood gives the height of the above as a little above high-water mark. (Ibid. vol. v. p. 110.) The following are from Mr. Carne's paper (op. cit.). b. At Carn Silver, boulders and pebbles were found in the end of a cavern, 8 feet wide and 12 feet high, once probably filled with them. c. In St. Loy Cove, under 30 feet of Head of granitic stones in clay, pebbles and boulders were observed, 4 to 8 feet in thickness, 150 feet in length, and at their base at high-water mark. (Present beach composed of granite boulders.-W.U.) d. Boulders were also observed at Polwarnon (? Polguaruon) Cove, Lean Scath, Pednvounder Cove (near the Logan rock), and at the Land's End Hole, but their height above the sea is not given. e. Near Penberth on the east, I noticed a small patch of Head composed of brown loam with angular stones and angular and subangular boulders. 11. Land's End. a. In Whitesand Bay, near Carn Aire, the Head consists of angular and subangular fragments and boulders of granite in coarse light buffbrown granitic debris (growan), becoming browner and more loamy near the base. The present beach is composed of granite boulders. b. Between Creagle and Aire Points, Mr. Carne (op. cit.) observed 6 feet of boulders and pebbles under 30 feet of clay with granitic fragments. Base of boulder bed at about spring tide high water. c. On the south of the Nanjulian River (Carne, op. cit.) boulders and pebbles occur at 15 feet above high water. d. On the south of Pol Pry (op. cit.), a thin bed of boulders at 20 feet above high water. e. In an iron vein at Huel Oak Point (op. cit.) boulders were found at 8 feet above high water. 12. Pornanvon and Porth Just. a. In Pornanvon Cove Mr. Carne (op. cit.) noticed 2 boulder beds (in a matrix of calcareous sand, granitic gravel and clay), separated by a mass of solid granite. The westernmost bed being 4 chains long, 10 feet thick, and overlain by 60 feet of granitic debris; that on the east was found to be 9 chains long, 20 feet in maximum thickness, and surmounted by 20 to 50 feet of granitic debris. The boulders vary in size from that of a hazel nut to 3 feet in diameter; no large slate boulders were noticed. The base of the deposit is about the level of very high spring tides. At Porth Just Mr. Carne found boulders at 15 feet above high-water mark. b. Mr. Henwood (T. R. G. S. Corn. vol. v. p. 13) mentioned the occurrence of rounded stones of granite, from the size of a nut to 2 or 3 feet in diameter, with a few slate pebbles, and with granitic sand filling the interstices, at from 15 to 20 feet above high water, at Porth Just and Pornanvon. He says that an adit at Wheal Besans Lode, Little Bounds Mine, was driven for several fathoms through one of these beds, which was found to be from 60 to 70 feet in thickness. (In this estimate the overlying Head was probably included.-W.U.) c. Miss Carne (T. R. G. S. Corn. vol. vii. p. 371) stated that the adit of a mine south of Kennal Point enters the cliffs under a mass of pebbles and boulders. 13. Cape Cornwall. a. In the south part of Priest Cove I noticed a few pebbles and subangular stones (one of granite), in olive-brown loam, and, occasionally, greyish sand, under 50 to 60 feet of Head, which presents a stratiform appearance through unequal distribution of fragments, and different tints. b. In a little cove just north of Cape Cornwall I observed the following section (Fig. 2) : Head. -Raised beach. FIG. 2.-Cape Cornwall on the North side. 1 Inch 24 Feet. ... Head, brown loam with numerous angular stones,containing ... ... 13ft. Oin. 5ft. Oin. Base of the deposit about 6 feet above high water. Boulders on the present beach. Rock platforms are visible at about high-water mark. c. In Porthleden Cove the following section was taken :— Head, brown loam with small angular pieces of quartz, containing small fragments of slate, and, occasionally, granite, 12 feet thick; upon yellowish-brown and brown loam with a few angular fragments; upon well-worn and subangular boulders with a few large pebbles, a few feet above high water. d. Mr. Godwin-Austen (Q. J. G. S. vol. vii. p. 121) notices the occurrence of granite pebbles, under yellowish clay, with large and small angular stones, and from 5 to 20 feet in thickness, at Creek Tor, in the parish of St. Just, Penrith. e. On the north of Cape Cornwall, Mr. Carne (T. R. G. S. Corn. vol. iii. p. 229) noticed a bed of slate boulders, 2 feet thick, and a chain in length, on greenstone at 10 feet above high water. The boulders were imbedded in clay and sand with small slate particles. 14. Pendeen Cove (op. cit.). Mr. Carne observed 3 feet of small pebbles in sand, made up of comminuted marine shells and pulverized granite, in one place capped by a bed of sand, overlain by 60 feet of Head. The base of the deposit is at about the level of spring-tide high water. The sand is in process of consolidation by iron oxide; it appears to have been blown from the beach into the interstices of the gravel. (To be continued in our next Number.) NOTICES OF MEMOIRS. I.-GEOLOGICAL NOTES ON WESTERN AFRICA. By Dr. O. LENZ.1 [Communicated by Count MARSCHALL, F.C.G.S.] 1. The Gabbro of Monrovia.-Gabbro appears near Monrovia in the form of irregularly fissured, isolated massives, rising above hills covered with the richest tropical vegetation. In its fresh condition it is dark green, distinctly granular, without any traces of schistose or porphyritic texture. Microscopical investigation proves a light-grey plagioclase to be its chief component, together with light-yellow tabular crystals of diallage, and interspersed particles of titanate of iron. The presence of serpentine also is probable, although not ascertained by positive observation. 2. Polished Rocks in the Beds of Rivers.-Several of the WestAfrican Rivers, opening into the Atlantic, force the lower portion of their course through a low and long chain of crystalline schists and quartzites, striking N.-S. Violent rapids, cataracts, and cascades, especially in the Congo and Ogowe, are serious obstacles to navigation. The rocks in the bed and on both banks, as far as they come in contact with the waters, are covered with a thin darkbrown varnish-like crust of extremely thin lamellæ of oxyd of iron, whose uppermost surface, continually exposed to the action of water, 1 [Proceed. Imper. Geol. Instit. Vienna, January, February, and March, 1878.] See also GEOL. MAG. Dec. II., Vol. IV. p. 27, and Vol. V. p. 312. |