REPORT OF THE DIRECTOR IQlJ 12$ 



pression, and develop cleavage lines along which they have a tend- 

 ency to split. This is termed foliation. There is. a wide difference 

 in the development of foliation in the various rocks subjected to the 

 same compressive forces. The impure sandstones and shales of the 

 Grenville series display foliation on such a fine and even scale that 

 they split with considerable ease and form what are known as schists. 

 These lines of cleavage may be entirely independent of the original 

 bedding planes of the rocks, which under the stresses to which they 

 have been subjected have become destroyed. 



The igneous rocks have been subjected to similar stresses but 

 probably have not passed through so many mountain-making 

 epochs as have the Grenville; hence they show less foliation. 

 Another reason why they show less cleavage is that the composition 

 is different. The anorthosite, being to a very large degree composed 

 of one mineral, shows less structural change from the rearrange- 

 ment of the individual crystals, but it is easily seen, however, that the 

 rock has been mashed and crushed. 



Folds 



In such regions as the mountains of Pennsylvania, where the 

 surface rocks are largely sedimentary in origin, it is a comparatively 

 simple matter to observe the folded condition of the rocks. The 

 situation is very different in this area. Nevertheless, considerable 

 folding has occurred between the time of solidification of the major 

 eruptives and the appearance of the diabase dikes. These minor 

 evidences of igneous activity in well-preserved state furnish good 

 evidence of the time of the major faulting and folding. Very minor 

 folding may, however, have occurred since that time in the Adiron- 

 dacks, but it is very slight. In Vermont the folds are much more 

 marked. 



Faults 



As the result of the tremendous strains and stresses to which' the 

 rock masses were subjected, they frequently fractured, those on one 

 side of the break slipping up, down or laterally from their former 

 position. These fractures with their resultant displacement are 

 termed faults. 



In the Adirondack region the results of faulting furnish conspicuous 

 physiographic features. In the immediate neighborhood of Lake 

 Clear it is difficult to recognize the existing fault lines as the country 

 is covered with sand and gravel to such an extent that their identi- 

 fication must depend upon topographic rather than upon structural 



