146 Geological Society. 



consists of hard green and purple grits and slates contrasting strongly 

 with the former in structure and appearance. 



These differences the author considered were due to deposition in 

 distinct basins lying on either side of an Archaean ridge of crystalline 

 rocks which ranged probably from Scandinavia through the central 

 highlands of Scotland, and included the north and west of Ireland, 

 with the counties of Donegal, Derry, Mayo, Sligo, and Galway — in 

 all of which the Cambrian beds were absent, so that the Lower 

 Silurians repose directly and unconformable- on the crystalline rocks 

 of Lauren ti an age. 



As additional evidence of the existence of this old ridge, the author 

 showed that when the Lower Silurian beds were in course of for- 

 mation, the Archaean floor along the west of Scotland must have 

 sloped upwards towards the east; but he agreed with Professor 

 Ramsay that the crystalline rocks of the Outer Hebrides formed 

 the western limit of the Cambrian area of deposition, and that the 

 basin was in the form of an inland lake. 



On the other hand, looking at the fossil evidence both of the 

 Irish and "Welsh Cambrian beds, he was of opinion that the beds of 

 this basin were in the main, if not altogether, of marine origin, and 

 that the basin itself had a greatly wider range eastward and south- 

 ward, the old Archaean ridge of the British Isles forming but a 

 small portion of the original margin. 



3. " The Devono-Silurian Formation." By Prof. E. Hull. LL.D., 

 F.R.S., F.G.S. 



The beds which the author proposed to group under the above 

 designation are found at various parts of the British Isles, and to a 

 slight extent on the continent. The formation, however, is emi- 

 nently British, and occurs under various local names, of which the 

 following are the principal : — 



EXGLAXD AND WAXES. 



Devonshire. — The Foreland Grits and Slates lying below the 

 Lower Devonian beds (" Lynton Beds"). 



Welsh Borders. — " The passage-beds " of Murchison, above the 

 Upper Ludlow Bone-bed, and including the Downton Sandstone, 

 and rocks of the Ridge of the Trichrug. These beds form the con- 

 necting link between the Estuarine Devonian beds of Hereford 

 (generally, but erroneously, called the " Old Red Sandstone ") and 

 the Upper Silurian Series. 



South-east of England (Sub-Cretaceous district). — The author 

 assumed, from the borings at Ware, Turnford, and Tottenham Court 

 Road described by Mr. Etheridge, that the Devono-Silurian beds 

 lie concealed between Turnford and Tottenham Court Road on the 

 south and Hertford on the north. 



Ireland. 

 South.— "The Dingle Beds," or " Glengariff Grits and Slates," 



