812 DYNAMICAL GEOLOGY. 



afford, at the ordinary pressure, nearly 45 cubic feet of steam to the cubic 

 foot of rock. There is no doubt, therefore, about enough moisture. 



The distribution of heat through the rocks without the aid of moisture 

 is impossible ; for heat travels but a short way into dry rock. A thickness 

 of two or three feet is sufficient to confine nearly all the heat of the hottest 

 furnace, and will make it safe to walk over liquid lavas. But let the walls 

 of the furnace be wet, and the heat will go through with a rush, for the 

 water becomes steam. 



4. Pressure. — Pressure, as already stated, is the chief source of the 

 movements by which a large part of the heat for metamorphism was pro- 

 duced. It has caused (1) a foliated structure in slates and other rocks, and 

 (2) minor changes in the texture of rocks. The first of these subjects is 

 treated under mountain-making ; the second, on page 321. 



In the following remarks, local metamorphism is first considered, and 

 then regional. 



Local Metamorphism. 



Local metamorphism, as above explained, makes changes in rocks in the 

 vicinity of the source of heat, as those of the walls of dikes. The results 

 are often called contact-phenomena, and any minerals formed, contact-minerals. 



The results of change along the walls of trap dikes in the Triassic areas 

 of eastern North America comprise minerals in the inclosing rock, in the 

 dike, or partly in both. They include crystallizations of epidote, tourmaline, 

 garnet, chlorite, quartz, hematite, and magnetite, besides various zeolites. 

 Garnets occur in the sandstone within a few yards of the ti*ap, and also in 

 rifts in the trap near its walls, and sometimes the latter are yellow "topazo- 

 lites of great beauty. Many square yards of the surface of a joint in the 

 trap of East E-ock, at New Haven, Conn., are thickly covered with garnets and 

 crystals of magnetite. At Rocky Hill, N. J., according to H. D. Rogers 

 (1840), the "baking" effects of a trap dike are distinct for a fourth of a mile 

 from the dike ; and, fifty feet off, a thin bed contains " kernels of pure epi- 

 dote," and cavities that are '' studded with crystals of tourmaline ; " and at 

 one place the latter crystals are half an inch in diameter. The sandstone, 

 when containing these minerals, has generally lost its usual red color and 

 become grayish-white to greenish, the green color coming sometimes from 

 the chlorite or epidote generated by the heat. 



The production of the metamorphic results, and the extent of the region 

 affected, has depended chiefly on the presence of moisture for conveying and 

 utilizing the heat. The sandstone walls of a dike may crumble into small 

 chips, because of the want of moisture there at the time of the eruption, 

 while in other places the rock becomes firmly consolidated. The presence 

 of steam is sometimes indicated by remains of the tubular channels through 

 which it rushed, their walls being bleached and penetrated with chlorite ; 

 and chlorite may occur, in some places near by, spangled with minute but 

 perfect crystals of hematite. 



