150 LYELL'S ELEMENTS OF GEOLOGY. 



Objections to the Metamorphic Theory. 



rently 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, as they are called, and slates 

 of the old red sandstone, in Forfarshire and other parts of Scot- 

 land, are so much charged with alkali, derived from triturated 

 felspar, that, instead of hardening when exposed to fire, they 

 melt readily 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 vari- 

 eties, and in almost all the interposed sandstones. These lami- 

 nated clays, marls, and shales might certainly, if crystallized, 

 resemble in composition many of the primary strata. 



There is also potash in the vegetable remains included in strata, 

 and soda in the salts by which they are sometimes so largely 

 impregnated, as in Patagonia. 



Another objection has been derived from the alternation of 

 highly crystalline strata with others having a less crystalline tex- 

 ture. 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 refractory as to be very slightly 

 affected by a degree of heat capable of reducing others to semi- 

 fusion. Nor should it be forgotten 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 metamorphic series. 



But it will be impossible for the reader duly to appreciate the 

 propriety of the term metamorphic, as applied to the strata hith- 

 erto called primary, until I have shown in the second part of this 

 work, that these crystalline strata have been formed at a great 

 variety of distinct periods. 



