Notices of Memoirs — Prof. Hull — Camhrian Beds. 507 



in Sutherlandsliire, in the district of Foinaven and Ben Arkle, where 

 the Lower Silurian beds rest directly on the Laurentiati Gneiss. 



5th. That Laureutian rocks may be recognized in other parts of 

 Ireland, as in the Slieve Garoph and Ox Mountains of Mayo and 

 Sligo, at Belmullet, and in West Galway north of Galway Bay, 

 where the rocks consist of red gneiss, hornblende rock, and schist, 

 etc., similar to those in Donegal; also possibly in Co. Tyrone, as 

 suggested by Mr. Kinahan. 



11. — Observations on the two Types of Cambrian Beds of 

 THE British Isles (the Caledonian and Hiberno-Cambrian), 

 AND the Conditions under which they were respectively 

 deposited. By Professor Edward Hull, LL.D., F.E.S., etc. 



N this paper the author pointed out the distinctions in mineral 

 character between the Cambrian beds of the North-West 

 Highlands of Scotland, and their assumed representatives in the East 

 of Ireland and North Wales and Salop. In the former case, which 

 included the beds belonging to the " Caledonian type," the formation 

 consists of red or purple sandstones and conglomerates ; in the 

 latter, which included the beds belonging to the "Hiberno-Cambrian 

 type," the formation consists of hard green and purple grits and 

 slates, contrasting strongly with the former in structure and ap- 

 pearance. 



These differences, the author considered, were due to deposition in 

 distinct basins, lying on either side of an Archeean ridge of crystal- 

 line 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 Silurian repose directly and unconformably on the crystalline 

 rocks of Laurentian age. 



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

 author showed that when the Lower Silurian beds were in course 

 of formation, the Archgean floor along the West of Scotland must 

 have sloped upwards towards the east, but he agreed with Professor 

 Kamsay, 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 southward, 

 the old Archaean ridge of the British Isles forming but a small 

 portion of the original margin. 



The beds included in the above represent the Llanbei'ris and 

 Harlech beds, those of the Longmynd and of St. Davids, in which 

 a remarkable primseval marine fauna had been discovered by Dr. 

 Hicks. 



