598 OBJECTIONS TO METAMOEPHIC THEOBY. [Ch. XXXV. 



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 de- 

 composition. But this reasoning proceeds on insufficient and apparently 

 mistaken data ; for a large portion of what is usually called clay, marl, 

 shale, and slate does actually contain a certain, and often a considerable, 

 proportion of alkali ; so that it is difficult, in many countries, to obtain clay 

 or shale sufficiently free from alkaline ingredients to allow of their being 

 burnt into bricks or used for pottery. 



Thus the argillaceous shales and slates of the Old Red sandstone, in 

 Forfarshire and other parts of Scotland, are so much charged with alkali, 

 derived from triturated felspar, that, instead of hardening when exposed 

 to fire, they sometimes melt into a glass. They contain no lime, but ap- 

 pear to consist of extremely minute grains of the various ingredients of 

 granite, which are distinctly visible in the coarser-grained varieties, and 

 in almost all the interposed sandstones. These laminated clays and shales 

 might certainly, if crystallized, resemble in composition many of the pri- 

 mary strata. 



There is also potash in fossil vegetable remains, and soda in the salts 

 by which strata are sometimes so largely impregnated, as in Patagonia. 

 But recent analysis may be said to have settled the point at issue, by 

 demonstrating that the carboniferous strata in England,} the Upper and 

 Lower Silurian in East Canada,^ and the clay-slates (of Cambrian date ?) 

 in Norway,§ all contain as much alkali as is generally present in meta- 

 morphic rocks. 



Another objection has been derived from the alternation of highly 

 crystalline strata with others having a less crystalline texture. The heat, 

 it is said, in its ascent from below, must have traversed the less altered 

 schists before it reached a higher and more crystalline bed. In answer 

 to this, it may be observed, that if a number of strata differing greatly 

 in composition from each other be subjected to equal quantities of heat, 

 there is every probability that some will be more fusible than others. 

 Some, for example, will contain soda, potash, lime, or some other ingre- 

 dient capable of acting as a flux ; while others may be destitute of the 

 same elements, and so refractoiy as to be very slightly affected by a de- 

 gree cf heat capable of reducing others to semi-fusion. Nor should it be 



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



t H. Taylor, Edin. New. Phil. Journ. voL i. 1851, p. HO. 



% Hunt, Phil. Mag. 4 ser. voL viL p. 237. 



§ Kyersly, Norsk, Mag. for Naturvidenp. vol viii. p. 172. 



