144 Geological Society. Prof. Sedgwick on the 



portion of the first section crosses the older beds (the Cambrian 

 system) which strike towards the N.E. The other section intersects 

 the upper series (Silurian system) which strike towards the N.W., 

 passing (in some places unconformably) round the beds of the older 

 system. From a consideration of the whole evidence the rocks are 

 grouped in the ascending order, as follows : — 



1. Chlorite slate, quartz rock, and mica slate of Anglesea and 

 Caernarvonshire. These are placed at the base of the section, and 

 form a distinct class ; and nothing is discovered in this part of the 

 section which is perfectly analogous with the Skiddaw slate, or first 

 Cumbrian group, to be after described. 



2. The old slate series of Caernarvonshire and Merionethshire, 

 alternating indefinitely with bands of porphyry and felspar rock : 

 the group is of enormous but unknown thickness, and is bent into 

 great undulations, the anticlinal and synclinal lines of 'which are 

 parallel to the strike of the chain. Through wide tracts of country 

 it is without fossils ; but at Moel Hebog, Snowdon, and Glider Fawr, 

 encrinites, corals, and a few species of bivalves have been discovered 

 in it. It ends with the calcareous beds which range from Bala to 

 the neighbourhood of Dinas Mowddy. This is called the Lower 

 Cambrian group. 



3. The next group (the Upper Cambrian group) commences with 

 the fossiliferous beds of Bala, includes all the higher portion of the 

 Berwyns, and all the slate rocks of South Wales which are below 

 the Silurian system. Its slate beds are less crystalline, and its 

 general structure is more mechanical, than the preceding group, and 

 it contains incomparably more fossils, which (though there are many 

 extensive portions of the group without fossils) are disseminated 

 through the more calcareous beds in great abundance. Many of 

 the fossils are identical in species with those of the lowest divisions of 

 the Silurian system, nor have any true positive zoological characters 

 of the group been well ascertained. 



In many parts of South Wales it is separated from the Silurian 

 system by great faults and derangements of the strata, marked by a 

 broad band of rotten non-fossiliferous schist. At the north end 

 of the Berwyn chain it appears to pass by insensible gradations 

 into the lower division of the Upper system (the Caradoc sand- 

 stone). 



4. The last natural group (the Silurian system). For all details 

 respecting this system the author refers to the abstracts of Mr. Mur- 

 chison's papers, and to his published works. 



The author then describes a series of sections : — 



(1.) East of the Berwyns, in which the Caradoc sandstone is 

 finely developed ; containing the Llandeilo flagstone and other cha- 

 racteristic calcareous and shelly bands. 



(2.) The sections north of the Berwyns, connecting Montgo- 

 meryshire with Denbighshire. The ascending series derived from 

 these sections is described as follows : — 



(1.) A series of beds several thousand feet in thickness, and at 

 the north end of the Berwyns apparently forming a passage 



