Reports and Proceedings — Geological Society of London. 43 



middle limb. The movements in which the Doncaster earthquake 

 originated presented a slight variation in this order. They consisted 

 of successive but continuous displacements, first of the south-western 

 arch, then of the middle limb, and finally of the north-eastern arch. 



II.— December 6th, 1905.— J. E. Marr, Sc.D., F.E.S., President, in 

 the Chair. The following communications were read : — 



1. " The Physical History of the Great Pleistocene Lake of 

 Portugal." By Professor Edward Hull, LL.D., F.R.S., F.G.S. 



The formations bordering the lower banks of the Tagus near 

 Lisbon are arranged by the author in the following order of 

 succession : — 



6. Eecent and Quaternary. Alluvia of the Tagus. 



*) T AOTTs ' (■ Marls with if/;n«(ga (Lacustre superior) . 



( Sauds and gravel. 



4. Post-Pliocene and ) JSTot represented, unless by some laud- 



Pliocene. ( glacial beds due to elevation. 



3. Miocene. ' Almada Beds.' Calcareous marls and lime- 



stones, with marine fossils. 



2. Eocene (?) . Uuf ossiliferous sands and gravels (Lacustre 



inferior) . 



1. Upper Cretaceous. Hippurite Limestone. 



, A description is given of the Lacustre superior ; the Almada 

 Beds are considered to be Miocene, and as the Pliocene is not 

 represented, except possibly by certain glacial deposits, the author 

 considers that that period was one of great uplift, when the 

 suboceanic gorge, an extension of the present course of the River 

 Tagus, was excavated. The margin of the lake was probably 

 formed by the granite of Das Vargans and Cunheira. There is 

 evidence that the general level of the lake-bed was nearly that of 

 the outer sea, and that the sea-waters gained occasional access to the 

 lake during the earlier stage of its formation. The lake was 

 eventually drained by the channel cut by the Tagus at the harbour 

 of Lisbon, upon the elevation of the land to about its present level. 



2. " The Geological Structure of the Sgurr of Eigg." By Alfred 

 Harker, M.A., F.R.S., F.G.S. 



The pitchstone which forms the Sgurr of Eigg is a massive sheet, 

 some 400 feet thick, reposing with discordance upon the succession 

 of alternating basalts and dolerites which make up the greater part 

 of the island. The lower surface of the pitchstone is irregularly 

 undulating, and in two places fragraental accumulations are seen 

 immediately beneath it. The generally received interpretation 

 regards the pitchstone as a lava-flow, or series of flows, occupying 

 an old river- valley excavated in the basalts, and the fragmental 

 deposits have been regarded as river-gravels of the pitchstone age. 

 This is the view put forward by Sir Archibald Geikie. 



After a detailed survey of the ground, the author finds it 

 impossible to accept this view, and he gives reasons for considering 

 the pitchstone to be intrusive. The form of its base, as mapped 

 out, does not seem to be reconcilable with that of a river-valley. 



