Ch. xxxy.] objections to metamokphic theory. 74.5 



shales, and slates do not, these last being the result of the decomposi- 

 tion of felspathic rocks, from which the alkaline matter has been ab- 

 stracted during the process of decomposition. 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 actu- 

 ally contain a certain, and often a considerable proportion of alkali ; 

 so that it is difficult, in many countries, to obtain clay or shale suf- 

 ficiently 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 appear 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 primary 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,f and the 

 clay-slates (of Cambrian or Laurentian date ?) in Norway,! all contain 

 as much alkali as is generally present in metamorphic 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, or hydrothermal action, there is every 

 probability that some will be much more fusible or soluble than 

 others. Some, for example, will contain soda, potash, lime, or some 

 other ingredient capable of acting as a flux or solvent ; while others 

 may be destitute of the same elements, and so refractory as to be 

 very slightly affected by the same causes. Nor should it be forgot- 

 ten that, as a general rule, the less crystalline rocks do really occur in 

 the upper, and the more crystalline in the lower part of each meta- 

 morphic series. 



Moreover, metamorphism must often begin to exert its force long 

 after the strata have assumed a vertical position, and it may then act 

 locally or within limited areas, and will be as likely to affect the 



* H. Taylor, Edin. New Phil. Jo-urn., vol. I., 1851, p. 140. 



•f- Hunt, Phil. Mag., 4th ser., vol. vii. p. 237. 



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



