106 Correspondence. [Feb., 



25,000 or 30,000 feet thick. It is naturally divided at least into 

 four groups, as follows: 



1. Lower Protozoic group. Lower part without fossils j upper 

 part contain lingula antiqua in abundance and fucoids, trilobites, 

 etc. 



2. Orthidian group. Numerous shells of the genus Orthis, 

 trilobites four to six species, corals, encrinites. The species as- 

 cend to the base of the upper Silurian rocks of Murchison. 



3. Enormous development of trappean rocks; stratified masses 

 sometimes containing organic remains (the scraalstein of German 

 Geologists,) enormous beds of roofing slate, &c. &c. The shells 

 abound here and there, and suddenly disappear on the line of strike, 

 at least for miles, and again reappear. 



4. Slates with occasional bands of contemporaneous porphyry, 

 &c., — generally more or less calcareous — three or four regular 

 bands of limestone (Bala limestone) thickness of these very greaL 



In addition to the species of No. 2, many new species enter — 

 some (such as arsterias and ophiura) are peculiar to this group. 



4. Cambro-Silurian Group. Llandielo, Caradoc: see Silurian 

 system for fossils. 



Here we have many fossils of the groups two and three, but we 

 have several which are /jecw/z'ar, and among them are abundant 

 such as pentamerus, Isevis and oblongus, also many fossils of the 

 Wenlock shale which are not found in the lower groups. Again; 

 the mineral character is peculiar — we have many beds of sand- 

 stone, sometimes coaree and passing into a conglomerate. 



In this scheme the classification and nomenclature are both 

 natural, I mean so far as regards this island, but by no means 

 apply to your country. Let each country (as 1 said before) be 

 worked out in its own evidence. 



The Llandielo and Caradoc are one formation, not two forma- 

 tions. They replace one another, but the Llandieilo is merely a 

 local development of a singular calcareous flag, and is compara- 

 tively of little value in general classification. 



It is not difficult to recognize in this scheme, of the distin- 

 guished Professor of Cambridge, the groups or systems of the rocks 

 in this country. 



L Thus the first group embraces at least a part of the Taconic 

 system, which seems to be more perfectly developed in this coun- 

 try than in l^ngland and Wales, and which has not suffered by 

 disturbance as there; in consequence of which, the base lines of 

 the two systems are apparently obliterated there, while here they 

 are preserved; being mdicated along the Hudson river, and Lake 

 Champlain by a band of limestone which is known as the calcif- 

 erous sandstone, and which rests upon the Taconic slate along 

 this range of country. 



