7\->r)'iJoni((n liocktt of the hie of Bum. 545 



"SlsLVch Uth.— Prof. Charles Lapworth, LL.D., F.K.S., 

 rresident, in the Cliair. 



The following communications were read : — 

 1. ' Petrological Notes on Rocks from Southern Abyssinia, 

 collected bj- Dr. lleginald Koettlitz.' By Catherine A. Raisin, D.Sc. 



The specimeus described in this paper were collected by 

 Dr. Koettlitz on an expedition (in 1898-99), starting from JBerbera, 

 westward through Somaliland and Southern Ab3'ssinia, and turning 

 northward to the Blue Xile. The paper gives petrological notes on 

 the different classes of rocks represented. The crystalline rocks 

 include granite, gneiss, and hornblende-schist or foliated diorite, 

 together with more basic types. They occur where the plateau 

 rises from the coastal plain, farther west underlying volcanic 

 rocks and sedimentary strata, in the south-west of Abyssinia, 

 and towards the Sudan. Some of the gneisses exhibit pressure- 

 effects, as if these older masses had been thrust up. The more basic 

 types include diabase, hornblende-gabbro, and one lustre-mottled 

 hornblende-pyroxenite, resembling a picrite. 



The sandstones (which are chiefly from Somaliland and the south- 

 east of Abyssinia) are sometimes compacted into quartzites, and 

 are often ferruginous. Some of the limestones are concretionary, 

 others are dolomitic, and several from different localities are 

 fossiliferous, containing foraminifera, calcareous algae, and, at 

 Ji^'iga Pass (which leads into Abyssinia), Turritella in great 

 numbers. 



The numerous specimens of volcanic rocks include one which is 

 practically a limburgite, many basalts (a few with olivine, and 

 some glassy), various less basic volcanic rocks, and several 

 pumiceous tuffs. But the most interesting are the phonolites 

 and allied rocks, containing nepheline, riebeckite, or other alkaline 

 minerals. They occur at several places, one being a volcanic hill 

 with a summit- crater. The authoress distinguishes several types 

 among these soda-bearing rocks, and compares two of them with 

 rocks of Central Abyssinia and of British East Africa respectively. 

 Thus the specimens here described may form a connecting-link 

 between the volcanic rocks of other East African localities. 



2. ' The Overthrust Torridoniau Eocks of the Isle of Bum and 

 the Associated Gneisses.' By Alfred Harker, Esq., M.A., E.R.S., 

 F.G.S. 



The Torridonian strata of Rum occupy all the northern part of 

 the island, together with a strip extending along the eastern coast, the 

 high ground in the south being made by plutonic rocks of Tertiary 

 age. The northern tract consists in general of sandstones having a 

 moderate dip to the north-west or west-north-west, and below these 

 there emerges on the east side a lower group composed of dark 

 shales. There are, however, two districts in which the strata are 

 highly disturbed and overthrust. One is a small area to the north- 

 west, on !Monadh Dubh, where a cake of thoroughly brecciated and 



Phil. Mag. S. 6. Vol. 6. No. 34. Oct. 1903. 2 N 



