182 THE WORLD, 



gravels, clays, limestones, coals, slates, and granites ; these t\v 

 classed, are the stratified, and unstratificd rocks. In the former, 

 are included those portions of the crust of the earth which ex- 

 hibit a sedimentary character, i. e., evidence of deposition through 

 the agency of water. It is supposed that all rocks were once de- 

 posited in this manner, but that the present crystaline form of 

 some of them is owing to heat, hence the tmstratified, are some- 

 times termed the igneous or the phitonic rocks; and the strata 

 which happen to intervene between them, being partly changed 

 in their character, yet not wholly so, are termed the metamorphic, 

 or transformed rocks; and those rocks resembling lavas scoriae, 

 and other substances, emitted by burning mountains, still in ac- 

 tivity", are called volcanic. When we assume that all the igneous 

 or plutonic rocks, such as granite, sienite, and the like, are of 

 sedimentary origin, we speak hypothetically, they are supposed 

 to be so ; for rocks, which are well known to present evidence of 

 aqueous origin, become crystaline, and lose the marks of stratifi- 

 cation under pressure, and by the influence of heat ; thus, chalk 

 has been converted into marble. Sir James Hall, exposed pound- 

 ed chalk to intense heat, under great pressure, and it was fused* 

 not into lime, but into crystaline marble ; even the shells inclosed 

 in the chalk underwent the same transmutation, yet preserved 

 their forms; and where ancient streams of lava have traversed 

 chalk, the latter invariably possesses a crystaline structure, and a 

 series of changes from a loose earthy deposit, to compact volcanic 

 lava, may be traced in numerous instances, so as to leave nd 

 doubt of the former aqueous origin, and the sedimentary deposit. 



The crystaline rocks, such as granite, sienite, porphyry, ser- 

 pentine, and greenstone, are generally termed the ancient or 

 earliest rocks, as they are uniformly found underlying all the 

 other strata ; they were hence, named hypo gent, under-lying, by 

 Mr. Lyell. It is now however, ascertained that they belong to no 

 particular age or epoch, exclusively ; for granite is found occur- 

 ring at comparatively modern, as well as ancient epochs, over- 

 laying the other strata precisely in the same manner as masses 

 of volcanic rock recently ejected, spread out upon the soil below. 

 The difference in character between the modern lavas, and 



