308 ON THE IGNEOUS METAMORPHOSIS [Ch. XIV. 



cannot be distinguished from some primary marbles. But, 

 great as these changes undoubtedly are, they may all be 

 attributable to the action of heat alone, except in the case of 

 dolomite, where a new element is supposed to be introduced ; 

 a supposition, however, which must be established on more 

 certain evidence before it be an acknowledged exception to 

 this general rule. 



Much light cannot at present be thrown on this subject by 

 a reference to existing chemical analyses ; but a table is sub- 

 joined, in the hope of directing attention to the importance 

 of this evidence. That experiments of this nature have not 

 hitherto been either numerous, or very accurate, is not 

 surprising ; for an indiscriminate analysis of these rocks offers 

 little inducement to the chemist, because the compounds are 

 very indefinite, on account of the constant variation in the 

 proportions of the constituent parts. But if the geologist 

 would carefully select important examples, minutely recording 

 all the circumstances of position and association, then the 

 assistance of the chemist would be a valuable acquisition. 

 Such an union has, indeed, already commenced ; and the ex- 

 ample of Turner and De la Beche must be hailed as an 

 important event in the history of our science. 



Imperfect as the following table is, it teaches us that the vol- 

 canic or igneous rocks generally contain an alkali, whilst the 

 derivative slates are devoid of this substance ; indeed, we might 

 conclude, a priori, that all rocks composed of earthy sub- 

 stances, derived from the decomposition of crystalline form- 

 ations, would be so circumstanced, since the alkali is always 

 extracted from such during chemical disintegration. 



