744 ORIGIN OF METAMORPHIC STRUCTURE. [Oh. XXXV. 



metamorphic texture ; the anthracite having been nearly transmuted 

 into that state of pure carbon which is called plumbago or graphite.* 



It has been remarked by M. Delesse that the minerals developed in 

 hypogene limestone vary according to the degree of metamorphism 

 which the rock has undergone. Thus, for example, where the struc- 

 ture is but slightly crystalline, talc, chlorite, serpentine, andalusite, and 

 kyanite are commonly present ; where it is more highly crystallized, 

 garnet, hornblende, wallastonite, dipyre, couzeranite, and some others 

 appear ; and, lastly, where the crystallization is complete, there are 

 found, in addition to many of the above minerals, felspar, especially 

 those kinds which are richest in alkali, together with mica. The 

 same author observes that, as calcareous deposits usually contain some 

 aluminous clay, so we may naturally expect to meet with silicates of 

 alumina in crystalline limestone ; such silicates, accordingly, are fre- 

 quent, and occasionally even pure alumina crystallized in the form of 

 corundum.f 



Mr. Dana has suggested that the phosphoric acid of phosphate of 

 lime, and the fluor of fluor-spar, so often met with in crystalline lime- 

 stones, may have been derived from the remains of mollusca and other 

 animals; also that graphite (which is pure carbon in a crystalline 

 form, with or without admixture of alumina, lime, or iron) may 

 have been derived from vegetable remains imbedded in the original 

 matrix. 



The total absence of any trace of fossils has inclined many geologists 

 to attribute the origin of the crystalline strata to a period antecedent 

 to the existence of organic beings. Admitting, they say, the oblitera- 

 tion, 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 living 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 nonfossiliferous, 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 con- 

 vertible. J The " primary " schists, it is said, usually contain a consider- 

 able proportion of potash, or of soda, which the secondary clays, 



* See Lyell, Quart. Geol. Journ., vol. i. p. 199. 



f Delesse, Bulletin Soc. Geol. France, 2e serie, torn. ix. p. 126, 1851. 



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



