224 



slates of Scotland and Ireland, and of various parts of the conti- 

 nent, &c. 



It is stated that the great difficulty is to draw clear and consistent 

 lines of demarcation between these great primary divisions. They 

 pass into one another, and interchange some fossil species, especially 

 near their limits. Were it not for the magnificent sections of the old 

 red sandstone in the British Isles, and also in the north-eastern 

 parts of Europe (as now shown by Mr. Murchison and his fellow 

 labourers), the author asserts, that the second and third divisions 

 ■would probably be confounded and eventually pass under one common 

 name (including Upper Silurian and Devonian). In Belgium and the 

 Rhenish provinces the demarcation between them is quite arbitrary. 

 We there find Goniatites and long- winged Spirifers and other organic 

 types (first supposed to characterize the " Devonian system") abun- 

 dant in the Silurian rocks : and there is nothing in the physical 

 structure of those countries to suggest the separation of these two 

 divisions. By help of the British sections, combined with new facts 

 brought to light in Russia and America, the second division must 

 however maintain its place. 



Between the third and fourth divisions there appears to be a much 

 better marked separation, both physically and zoologically, than be- 

 tween the others. These two divisions (at least in North Wales) 

 diflfer in structure, interchange hardly any fossil species, and through 

 large districts are unconformable. Hence they belong to two systems, 

 and not to one, if the vforA system be used in a definite sense, and be 

 applied to the successive divisions (such as the Devonian). To avoid 

 incongruity of language, the author uses the word system in a ge- 

 neral sense ; and under the name Palceozoic System describes the 

 whole series of formations comprehended under the four divisions 

 above described. In this great descending series the " Silurian 

 system" (in the sense in which the words were first used) stands 

 in the place of the third, and the upper part of the fourth primary 

 division. 



