172 Notices of Memoirs — Dr. 0. Lenz — Geology of W. Africa. 



Base of the deposit about 6 feet above high water. Boulders on the 

 present beach. Bock 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 frag- 

 ments ; 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 oc- 

 currence 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. B. 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 o:f mbmoibs. 



I. — Geological Notes on Western Africa. By Dr. 0. Lenz. 1 



[Communicated by Count Makschall, 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 Bocks in the Beds of Bivers. — Several of the West- 

 African Bivers, 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 navi- 

 gation. The rocks in the bed and on both banks, as far as they 

 come in contact with the waters, are covered with a thin dark- 

 brown varnish-like crust of extremely thin lamella? 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. 



