48 Analyses of Rocks. 



might seem to be truly granitic ; but when the detailed 

 geology of the country has been worked out, they are 

 found to follow all the great anticlinal and synclinal 

 folds of metamorphosed strata that have also in a minor 

 way been intensely contorted. The same is the case 

 in parts of the Alps. 



I have already stated that if we chemically analyse 

 a series of specimens of clays, shales, and slates, often 

 more or less sandy, together with various gneissic rocks 

 and granites, it is remarkable how closely the quantities 

 of their ultimate constituents, in many cases, approach 

 to each other. They are never identical, while yet the 

 resemblance is close, as close indeed as it may be in 

 two specimens of the same kind of sandy shale or slate. 

 In all of them silica would form by far the largest 

 proportion, say from 60 to 70 per cent. ; alumina would 

 come next, and then other substances, such as lime, 

 soda, potash, iron, &c., would be found in smaller 

 varying proportions ; and what I now wish to express 

 is, that the distinct minerals developed in the gneiss, 

 such as felspar, mica, garnets, &c., were not new sub- 

 stances introduced into the rock, by contact with 

 granite, or by any other process, but were all developed 

 under the influence of metamorphism from materials 

 that previously existed in the strata before their meta- 

 morphism began, aided by hydrothermal action due 

 to the presence of heated alkaline waters deep beneath 

 the surface of the earth. Through some process, in 

 which heat played a large part, the rock having been 

 softened, and water present in most rocks underground 

 having been diffused throughout the mass and heated, 

 chemical action was set up, and the substances that 

 composed the shale or slate, often mingled with silicious 

 sandy material, were enabled more or less to re-arrange 



