3-34 Reports and Proceedings — Geological Society of London. 



attains a height of 16,500 feet. The western slope is at an angle 

 of 22° ; the eastern slope at about one of 4°. The authors describe 

 sections across the ridge at right angles to its trend. These show- 

 that Ruwenzori is not volcanic, nor is it a simple massif o£ diorite. 

 Epidiorite occurs only as banded sheets in the schists on the flanks 

 of the mountain, and is not the central rock of the ridge. The strike 

 of the flanking schists seems to run concentrically round the ridge 

 as though the centi-al rock were intrusive into them. The highest 

 rock collected, a coarse-grained granite or granitoid gneiss, may be 

 an intrusive igneous rock, but it may be part of the old Archaean 

 series faulted up ; there is nothing in its microscopical characters to 

 separate it from the Archaean rocks, and the authors think it 

 probable that this rock was raised into its present position by faulting. 

 In this case Euwenzori is simply composed of an orographic block 

 or " scholl," which was at one time probably jDart of a wide plateau 

 of Archgean rocks. 



There is abundant evidence of volcanic action around Euwenzori, 

 for the plains, especially to the east and south-east, are studded 

 with small volcanic cones, arranged on lines which radiate from 

 Euwenzori. 



It is afiirraed that evidence points to the former occupation of the 

 Nyamwamba, Mubuka. and Batagu vallej's by glaciers, roches 

 moutomiees of typical character having been noted in the two 

 former valleys. 



The country round Euwenzori consists of rocks which may be 

 conveniently grouped into two series — one composed of gneisses and 

 schists, and the other of non-foliated sediments. The former (the 

 Archaean series) are of the type that has an enormous extension in 

 Equatorial Africa, and forms the main plateau on which all the 

 sediments and volcanic rocks have been deposited. 



The sedimentary rocks are probably Palaeozoic, possibly pre- 

 Carboniferous, but in the absence of fossils it would be unsafe to go 

 beyond this statement. 



3. " On Overthrusts of Tertiary Date in Dorset." By A. Strahan, 

 Esq., M.A., F.G.S. (Communicated by permission of the Director- 

 General of the Geological Survey.) 



The results given in this paper were obtained during a re-survey 

 of South Dorset on the 6-inch scale. The disturbances can be 

 divided into two groups, — the one being mainly of Miocene date, 

 and the other of intra-Cretaceous (between Wealden and Gault) age. 

 The former includes the Isle of Purbeck fold (which is the continua- 

 tion of the Isle of Wight disturbance), the Eingstead fold, the 

 Chaldon and Eidgeway disturbances, and the Litton Cheney fault. 

 In the latter are placed the anticline of Osmington Mill, the syncline 

 of Upton, and a part of the anticline of Chaldon ; farther west the 

 Broadway anticline and Upway syncline, a fault at Abbotsbury, and 

 many other folds come into the same group. These earlier move- 

 ments led to the well-known unconformity at the base of the Upper 

 Cretaceous rocks. 



The Isle of Purbeck fold is accompanied by a large thrust-fault, 



