654 THE POPULAR SCIENCE MONTHLY. 



blende are well defined, and commonly have a more or less paral- 

 lel arrangement ; here and there are bands in which these minerals 

 are more abundant than elsewhere. The quartz and the feldspar 

 are granular in form ; the boundaries of these minerals are not 

 rectilinear, but curved, wavy, or lobate ; small grains of the one 

 sometimes appear to be inclosed in larger grains of the other. 

 Though the structure of this rock has a superficial resemblance to 

 that of a granite of similar coarseness, it differs from it in this 

 respect, as we can see from the next instance, a true granite, where 

 the rectilinear outline of the feldspar is conspicuous. Here, then, 

 is one of our problems. This difference of structure is too general 

 to be without significance. What, then, does it mean ? Among 

 the agents of change known to geologists, three are admittedly of 

 great importance : these are water, heat, and pressure. The first 

 effect of pressure due to great earth-movements is to flatten some- 

 what the larger fragments in rocks, and to produce in those of 

 finer grain the structure called cleavage. This, however, is a 

 modification mainly mechanical. It consists in a rearrangement 

 of the constituent particles ; mineral changes, so far as they occur, 

 being quite subordinate. But in certain extreme instances the 

 latter are also conspicuous. From the fine mud, generally the 

 result of the disintegration of feldspar, a mica, usually colorless, 

 has been produced, which occurs in tiny flakes, often less than 

 one hundredth of an inch long. In this process a certain amount 

 of silica has been liberated, which sometimes augments pre-exist- 

 ing granules of quartz, sometimes consolidates independently as 

 micro-crystalline quartz. Simultaneously carbonaceous and fer- 

 ruginous constituents are converted into particles of graphite or 

 of iron oxide. As to the effects of pressure when it acts upon a 

 rock already crystalline, there are, as it seems to me, differences in 

 the resultant structures which are dependent upon the mode in 

 which pressure has acted. They are divisible into two groups ; 

 one indicating the result of simple direct crushing, the other of 

 crushing accompanied by shearing. In the former case, the rock- 

 mass has been so situated that any appreciable lateral movement 

 has been impossible ; it has yielded like a block in a crushing- 

 machine. In the latter, a differential lateral movement of the 

 particles has been possible, and it has prevailed when (as in the 

 case of an overthrust fault) the whole mass has not only suffered 

 compression, but also has traveled slowly forward. Obviously, 

 the two cases can not be sharply divided, for the crushing up of 

 a non-homogeneous rock may render some local shearing possi- 

 ble. Still, it is important to separate them in our minds, and we 

 shall find that in many cases the structure, as a whole, like the 

 cleavage of a slate, results from a direct crush ; while in others 

 the effects of shearing predominate. The latter, accordingly, ex- 



