6o8 STUDIES FOR STUDENTS 



extreme change the genesis of the rocks and their structures 

 are determined with reasonable certainty. 



In the eastern region the areas in which the sedimentary rocks 

 have gone through the process above described are more exten- 

 sive than those of igneous rocks, but all steps of the process 

 have also affected extensive areas of igneous rocks. Examples 

 of the latter are the pre-Cambrian granitoid gneiss of the Green 

 Mountains, and, more extensive than this, the great areas of 

 ancient plutonic and volcanic rocks of the Blue Ridge and Pied- 

 mont Plateau. 



It is believed that the regularly banded and laminated Lau- 

 rentian gneisses which have an isoclinal dip over great areas in 

 Canada, along the Madison Canyon in southwestern Montana, 

 and in other regions, may be explained by the same processes 

 completely carried out as applicable to the Appalachians. It is 

 not asserted whether the original rocks in these regions were 

 igneous or aqueous. The general drift of opinion in recent years 

 is in favor of the former origin. 



RELATIONS OF CLEAVAGE AND FISSILITY TO STRATIGRAPHY. 



Cleavage or fissility may be developed in one set of beds 

 and not in another set of beds in the same set of formations. 

 Secondary structures develop readily in a shale, less readily 

 in a fine grit, still less readily in a limestone, and perhaps 

 with the least readiness in a quartzose sandstone, quartzite, 

 or conglomerate. As a consequence of this, a shale between 

 beds of limestone may take on a thoroughly cleaved or fissile 

 character, the cleavage stopping abruptly at the beds of the 

 limestone. The same is true of layers of shale between beds of 

 grit, or sandstone, or quartzite, or conglomerate. In general, in 

 the strata coiistittiting a formatio?i, cleavage may develop hi the less 

 rigid beds and be absent or imperfect in the more rigid beds. In 

 such cases it has sometimes been assumed that the lower bed was 

 deposited and cleavage or fissility developed in it before the 

 superior bed was formed, the fact that a secondary structure 

 was not so ready to develop in the upper bed being ignored. 



