266 American Association. 



This phenomenou, however, simply shows that a great break or 

 local change in the sediments, took place in the S. W. of Ireland 

 which had no existence in the north east of Scotland, where the 

 Old Red or Devonian series is continuous. 



I cannot on this occasion enter into questions of detail concern- 

 ing the localities where the Upper Silurian strata pass upwards 

 with perfect conformity into the Old Red or Devonian rocks,) or 

 indicate other tracts ia Europe (notably in France and Spain) 

 where on the contrary the Upper Silurian is entirely omitted. In 

 regard to local dislocations, I particularly refer you to my com- 

 parison of the Old Rocks of the Thuringerwald and the Hartz.* I 

 will simply conclude this letter by calling your attention to what 

 is now seen to be the true method of comparing the Older 

 Palaeozoic or Silurian Rocks of distant regions. 



When that skilful and profound geologist, Mr. Barrande, pub- 

 lished in the course of last year his most instructive essay, entitled 

 " Parallele entre les depots siluriens de Boheme et de Scandinavie," 

 he showed how with an agreement in generic characters of the 

 fossils of each Silurian zone, thus indicating a general harmony, 

 there was a great contrast in the species of marine animals in each 

 of the countries compared. By applying this method in a differ- 

 ent sense, I may now say that when the Silurian rocks are viewed 

 in their extension through the same latitudes, a remarkable specific 

 agreement is clearly traceable. On the other hand the Silurian 

 fossils of Bohemia are in accordance with those of France and 

 Spain, or along another and distinct broad southern zone of the 

 same age. 



The Silurians of Scandinavia are of the British and American 

 type. In making known the description of the Silurian rocks of 

 Norway by Mr. Kejereslf f, I have recently shown how remarkable 

 is the persistence of the Lower Silurian types (even in sj)ecies)when 

 these rocks are followed from Scandinavia into the British Isles, 

 and to how great an extent this resemblance of type is preserved, 

 even v^hen the Atlantic is traversed, and that the same strata 

 m the crust of the globe are again met with in North America. 

 The occurrence in the south of Scotland of the Maclurea magna 

 of Hall, of the Isotelus gigas in Ireland, and of the fossils 

 of your calciferous sand-rock in our Scottish Highlands, are all 

 most satisfactory proofs that the order in Canada and the country 



* Quarterly Journal, Geological Societj, November, 1855. 

 t Journal Geological Society, about to be piiblished. 



