DESCRIPTIONS OP ROCKS. 413 



A. By cooling from fusion. The rocks thus made are called 

 Igneous or Eruptive rocks, as, for example, lavas or vol- 

 canic ejections, and all rocks that, like trap, have come up 

 melted through fissures in the earth's rocky crust. The 

 depth of the liquid source of such eruptions is unknown. 

 The fact that, at one epoch, material of the same kind has 

 sometimes been ejected at intervals along a band of country 

 a thousand miles in length, from northeast to southwest, as 



' on the Atlantic coast from Nova Scotia to South Carolina, 

 indicates considerable depth in such cases. They may be 

 older rocks melted over and thrust up to the surface ; but if 

 so, the remelted rocks were in many cases those situated deep 

 in the earth's crust, far below all the strata of its surface. 



B. By subjection to long-continued heat without fusion, 

 making metamorphic rocks. Through this means fragmen- 

 tal or sedimentary strata, over areas of thousands of square 

 miles, and many thousands of feet in depth, have been 

 simultaneously crystallized, turning the beds that were 

 originally made from sand, gravel, or mud, into granite, 

 gneiss, and other related rocks, and compact limestones 

 into marble. The rocks at the time of the change were 

 generally undergoing extensive mountain-making uplifts, 

 and it is supposed that the friction attending the movements 

 of the strata may have been an important source of heat for 

 the change or crystallization ; and that the diffusion of this 

 heat was due to the moisture which abounds in unaltered 

 sedimentary beds. Metamorphic strata retain their former 

 relative order of superposition, having been crystallized in 

 place, that is, without fusion. Where granite has been the 

 result, it is probable that the material was sometimes re- 

 duced to a pasty state, so that all lines of the original bed- 

 ding were obliterated ; but even in that case, the granite is 

 generally in the place occupied by the material before crys- 

 tallization. In other cases, including that of some granites, 

 there was not even this degree of approach toward the origi- 

 nal condition of the true eruptive rock. During the upturn- 

 ing, the rocks were much fractured, and the fissures so made 

 became filled with the materials of the adjoining or subjacent 

 rocks, through the aid of the heated moisture present, mak- 

 ing veins ; and such veins differ widely from those, called 

 dikes, that were made when the fractures descended to re- 

 gions of melted rock, so that the fissures became filled with 

 ejected material. 



