i;8 OUTLINES OF FIELD-GEOLOGY PART i 



Gradually the idea of following the same band among 

 the contorted masses of the schistose rocks will seem to 

 him more and more feasible; and he may at length be 

 induced seriously to make the attempt to unravel the 

 complicated geological structure of the region. 



In these investigations I have found that four points 

 deserve to be kept steadily in view, (i) The nature and 

 distribution of the minerals. (2) The varieties and 

 alternations of the rocks. (3) The direction of the pre- 

 valent foliation, and whether or not it coincides with 

 bedding. (4) The evidence of crushing, and the exist- 

 ence of thrust-planes. 



i. As I have just said, the minerals for which regions 

 of gneiss and schist are celebrated may be employed by 

 the stratigraphical geologist much as he would use fossils. 

 Besides their own beauty, they afford endless interest and 

 instruction in the light they cast upon the formation of 

 the rocks among which they occur, and in the problems 

 they present to us regarding mineral growth. But these 

 aspects must be studied chiefly in the laboratory, and 

 with the microscope. In field -geology, the observer 

 notes as many facts as strike his mind in connection with 

 theoretical speculation, but for his own work at the time 

 it is the association and distribution of the minerals 

 which are of prime importance. He will therefore watch, 

 as he traverses the mountains, under what circumstances 

 special minerals occur. Let us suppose for example, that 

 he encounters a rock containing the beautiful bluish- 

 grey mineral called kyanite. He carefully notes how it 

 occurs, with what other minerals associated, and in what 

 kind of rock. Wherever he comes upon a loose fragment of 



