484 British Association for the Advancement of Science'. 



passages from the lowest member of the " Silurian system" into the 

 underlying slaty rocks, now named by Professor Sedgwick the 

 " Upper Cambrian? 



This communication was illustrated by Ordnance Maps extending 

 over large parts of eleven counties, coloured in the field by Mr. 

 Murchison. 



Professor Sedgwick commenced by pointing out the imperfection 

 of the sections exhibited in the North of England, and some por- 

 tions of North Wales, in consequence of the entire want of conti- 

 nuity between the carboniferous series and the inferior schistose 

 groups. Some of the latter are fossiliferous both in Denbigh- 

 shire and Westmorland ; but in the interrupted sections of those 

 counties it is impossible to tell how many terms are wanting to com- 

 plete the series to the old red sandstone and carboniferous lime- 

 stone. In the country described by Mr. Murchison these difficulties 

 do not exist, and his sections have filled up a wide chasm in the 

 succession of British deposits. Professor Sedgwick then described 

 in descending order the groups of slate rocks, as they are seen in 

 Wales and Cumberland. To the highest he gave the name of Upper 

 Cambrian group. It occupies the greatest part of the chain of the 

 Berwyns, where it is connected with the Llandeilo flags of the Silu- 

 rian system, and is thence expanded through a considerable portion 

 of South Wales. In one part of its course it is based on beds of lime- 

 stone and calcareous slate ; but on the whole, it contains much less 

 calcareous matter than the Silurian system, and has fewer organic re- 

 mains. Beds of good roofing-slate occur, and a perfect slaty cleav- 

 age is often observed in it transverse to the stratification ; but other 

 parts of it are of a coarse mechanical texture. To the next inferior 

 group he gave the name of Middle Cambrian. It composes all the 

 higher mountains of Caernarvonshire and Merionethshire, and 

 abounds in fine roofing-slate> alternating with, and apparently pass- 

 ing into, irregularly interstratified masses of porphyry. Some por- 

 tions of it are coarse and mechanical, and it contains (for example, 

 at the top of Snowdon,) a few organic remains, and a few examples 

 of highly calcareous slates, but no continuous beds of limestone. 

 The same group, with the same mineral structure, and in the same 

 position, but without organic remains, is greatly developed in Cum- 

 berland. The Lower Cambrian group occupies the S. W. coast of 

 Caernarvonshire, and a considerable portion of Anglesea : it consists 

 chiefly of chlorite schist, passing here and there into mica schist and 

 slaty quartz rock, and contains subordinate masses of serpentine and 

 white granular limestone. It contains no organic remains. Beneath 

 the Middle Cambrian system (above described) there occurs in 

 Cumberland (for example, Skiddaw Forest,) a great formation of 

 dark glossy clayslate, without calcareous matter, and without organic 

 remains. It passes in descending order into chiastolite slate, mica 

 slate, hornblende slate, gneiss, &c, which rest immediately on gra- 

 nite. Whether the Lower Cambrian was to be placed on the exact 

 parallel of these masses in Skiddaw Forest, the Professor did not de- 

 termine. 



