PART I. CHAPTER XI. 



149 



Origin of Metamorphic Rocks. 



derived from siliceous sandstone, and compact quartz from the 

 same materials. Clay-slate may be altered shale, and granular 

 marble may have originated in the form of ordinary limestone, 

 replete with shells and corals, which have since been obliterated; 

 and, lastly, calcareous sands and marls may have been changed 

 into impure crystalline limestones. 



"Hornblende-schist," says Dr. MacCulloch, "may at first 

 have been mere clay ,* for clay or shale is found altered by trap 

 into Lydian stone, a substance differing from hornblende-schist 

 almost solely in compactness and uniformity of texture."* " In 

 Shetland," remarks the same author, " argillaceous-schist (or 

 clay- slate,) when in contact with granite, is sometimes converted 

 into hornblende-schist, the schist becoming first siliceous, and 

 ultimately, at the contact, hornblende-schist. "f 



The anthracite found associated with hypogene rocks may 

 have been coal ; for we know that, in the vicinity of some trap 

 dikes, coal is converted into anthracite. 



The total absence of any trace of fossils has inclined many 

 geologists to attribute the origin of crystalline strata to a period 

 antecedent to the existence of organic beings. Admitting, they 

 say, the obliteration, in some cases, of fossils by plutonic action, 

 we might still expect that traces of them would oftener occur in 

 certain ancient systems of slate, in which, as in Cumberland, 

 some conglomerates occur. But in urging this argument, it seems 

 to have been forgotten, that there are stratified formations of 

 enormous thickness, and of various ages, and some of them very 

 modern, all formed after the earth had become the abode of liv- 

 ing creatures, which are nevertheless in certain districts entirely 

 destitute of all vestiges of organic bodies. In some, the traces 

 of fossils may have been effaced by water and acids, at many 

 successive periods ; and it is clear, that the older the stratum, 

 the greater is the chance of its being non-fossiliferous, even if it 

 has escaped all metamorphic action. 



It has been also objected to the metamorphic theory, that the 

 chemical composition of the secondary strata differs essentially 

 from that of the crystalline schists, into which they are supposed 

 to be convertible.:^: The " primary" schists, it is said, usually 

 contain a considerable proportion of potash or of soda, which the 

 secondary clays, shales, and slates do not, these last being the 

 result of the decomposition of felspathic rocks, from which the 

 alkaline matter has been abstracted during the process of decom- 

 position. But this reasoning proceeds on insufficient and appa- 



* Syst. of Geol., vol. i. p. 210. t Ibid., p, 211. 



t Dr. Boase, Primary Geology, p. 319. 



