vi Rise of Petrography 275 



These researches started the study of ancient glaciation. 

 Buckland, Lyell, Darwin, Chambers and others took up 

 the question, and added to the evidence adduced by 

 Agassiz from his rapid traverses. At first the existence 

 of former glaciers in the valleys of Britain was the con- 

 clusion chiefly sought to be established. British geologists, 

 and indeed geologists generally, were for many years 

 unwilling to admit that not only the mountain-valleys, 

 but even the lowlands of the northern hemisphere were 

 at a late geological period buried under sheets of ice. 

 They preferred to call in the action of floating ice, without 

 perceiving that in so doing they involved themselves in 

 far more serious physical difficulties than those which 

 they sought to avoid. But for many years past the 

 teaching of Agassiz in all its essential elements has been 

 generally accepted, and his name is now enshrined in the 

 records of our science as that of the true founder of 

 glacial geology. 



I turn now to the petrographical department of 

 geological inquiry, as exhibiting the last great forward 

 stride which our science has taken. We have seen how 

 greatly geology and mineralogy were indebted to Werner 

 for his careful and precise definitions. The impulse which 

 he gave to the study of Petrography continued to show its 

 effects long after his time, more particularly in Germany. 

 Methods of examination were improved, chemical analysis 

 was more resorted to, and the rocks of the earth's crust, 

 so far as related to their ultimate chemical constitution, 

 were fairly well known and classified. Their internal 

 structure, however, was very imperfectly understood. 

 Where they were coarsely crystalline, their component 



