14 NEW YORK STATE MUSEUM 



load, it is thought, is compensated by a transfer of material in the 

 substratum in the opposite direction, which causes a sinking of the 

 overweighted part and a corresponding elevation of the lighter 

 areas. 



The adjustments, however occasioned, are accompanied by im- 

 portant results in regard to rocks. Near the surface these yield to 

 the strain by fracture, which may take the form of innumerable 

 division planes or joints that break up the masses into polygonal 

 blocks. Or again, there may be formed one or more great fractures 

 along which the rocks have undergone appreciable differential move- 

 ment with the production of crushed zones. These movements, if 

 sudden, are accompanied by earthquakes. The large fractures may 

 extend downwards for indefinite distances, affording ready channels 

 for the passage of igneous material toward the surface, and thus 

 are connected with volcanic action. They are frequently found 

 with a rilling of some igneous rock like trap or porphyry, marking 

 the site of former eruptions. 



Within the depths of the earth a point may be reached where the 

 rocks can not accommodate themselves by fracture under the stress 

 of cubical compression, but adjust themselves by plastic yielding or 

 flowage. The weight of the overlying load causes them to have a 

 certain mobility, although actually in a solid state. Under unequal 

 stress as developed by side thrusts, they tend to move by flowage 

 toward the direction of least pressure. The depth at which this 

 method of deformation becomes effective has been estimated by 

 calculation and experiment at from 6 to 12 miles, the latter being 

 perhaps the maximum for the very hard resistant rocks. The in- 

 fluence of this mechanical action is augmented by the heat incident 

 to the depth at which it takes place and no doubt also by occluded 

 waters and gasses which facilitate the solution and recrystallization 

 of the minerals. 



The characteristics that are thus produced in rocks by com- 

 pression within the earth's interior are quite different from those 

 originally inherent in either igneous or sedimentary types and 

 belong to the metamorphic class. Members of the latter, like most 

 igneous rocks, possess a crystalline development, each mineral hav- 

 ing crystallized acording to its definite habit, but there are differ- 

 ences in the arrangement of the minerals which is quite typical. 

 Instead of a uniform distribution that arises from the cooling of 

 an igneous magma, producing a homogeneous aspect, whatever 

 plane may be exposed to view, they show a parallel structure and 



