LECTUEE VI 



The Transition or Greywacke formation resolved by Sedgwick and 

 Murchison into the Cambrian, Silurian and Devonian systems 

 The pre-Cambrian rocks first begun to be set in order by 

 Logan Foundation of Glacial Geology, Agassiz Eise of 

 modern Petrography ; William Nicol, Henry Clifton Sorby 

 The influence of Lyell and Darwin Conclusion. 



THE determination of the value of fossils as chronological 

 documents has done more than any other discovery to change 

 the character and accelerate the progress of geological in- 

 quiry. No contrast can be more striking than the difference 

 between the condition of the science before and after that 

 discovery was made. Before that time, when the Wernerian 

 classification of the rocks of the earth's crust everywhere 

 prevailed, there was really little stimulus to investigate 

 these rocks in their chronological relations to each other. 

 They were grouped, indeed, in a certain order, which was 

 believed to express their succession in time, but their 

 identification from one country to another proceeded on 

 no minute study of their internal structure, their fossil 

 contents, or their tectonic relations. It was thought 

 enough if they could be placed in one or other of the 

 divisions of the Freiberg system. When an orthodox 

 disciple of Werner had relegated a mass of deposits to the 



