IS^G.] SEDGWICK ON THE FOSSIL SLATES OF N. WALES, ETC. 159 



Conclusion. 



By way of conclusion, and in confirmation of what has been just 

 stated, I may shortly touch on the principles by which we have 

 hitherto been guided in our nomenclature of the older stratified 

 British rocks*. In every country which is not made out by refer- 

 ence to a pre-existing type, our first labour is that of determining 

 the physical groups, and establishing their relations by natural sec- 

 tions. The labour next in order is the determination of the fossils 

 found in the successive physical groups ; and, as a matter of fact, 

 the natural groups of fossils are generally found to be nearly co- 

 ordinate with the physical groups — each successive group resulting 

 from certain conditions which have modified the distribution of 

 organic types. In the third place comes the collective arrangement 

 of the groups into systems, or groups of a higher order. 



The establishment of the Silurian system is an admirable exam- 

 ple of this whole process. The groups called Caradoc, Wenlock, 

 Ludlow, &c., were physical groups determined by good natural sec- 

 tions. The successive groups of fossils were determined by the sec- 

 tions ; and the sections, as the representatives of physical groups, 

 were hardly at all modified by any consideration of the fossils, for 

 these two distinct views of the natural history of such groups led to 

 coordinate results. Then followed the collective view of the whole 

 series, and the establishment of a nomenclature. Not only the 

 whole series (considered as a distinct system), but every subordi- 

 nate group was defined by a geographical name, referring us to a 

 local type within the limits of Siluriaf. At the same time the older 

 slate rocks of Wales (inferior to the system of Siluria) were called 

 Cambrian, and soon afterwards the next great collective group of 

 rocks (superior to the system of Siluria) was called Devonian, In 

 this way was established a perfect congruity of language. It was 

 geographical in principle, and it represented the actual development 

 of all our older rocks, which gave to it its true value and meaning. 



This language, having once become current, was applied to the 

 rocks of distant countries. Thus I have described the third, and 

 highest, collective group of the Cumbrian mountains (the Westmore- 

 land group) as the exact equivalent of the whole Silurian system — 

 the Coniston limestone exactly representing the Caradoc sandstone, 

 and the highest beds (between Kendal and Kirkby Lonsdale) repre- 

 senting the " Upper Ludlow rocks " and the " Tilestone." But in 

 every case in which we have made use of this language, we have 

 assumed that the rocks under notice had their true equivalents and 



* The following remarks may also be applied to our secondary rocks ; for the 

 principles on which the grouping and nomenclature of our secondary rocks was 

 first determined by Smith were in exact accordance with what is stated in the 

 text. 



t The same process of analysis and the same principles of nomenclature were 

 adopted by myself during my whole examination of North Wales, and are the 

 groundwork of every part of the classification offered in this paper. 



