CHAPTEE IY. 



THE NATURE AND ORIGIN OF CRYSTALLINE AND IGNEOUS ROCKS. 



THE newest water-formed rocks are similar in appearance to deposits 

 which, are now being deposited ; but the older strata have often under- 

 gone changes which have obliterated some of their original features 

 which were due to deposition, and have imparted characters which 

 sometimes make it difficult or impossible to discover from observation 

 that they were ever deposited in water at all. These changes aro 

 partly the consequence of the slow infiltration of water, which dis- 

 solves certain mineral constituents from one place or one rock, and 

 deposits them again elsewhere, sometimes as crystalline minerals, but 

 almost always in different mineral combinations ; and when a rock is 

 thus altered by the action of water, it may be said to be transformed. 

 Other changes of a more varied and important character result from 

 the action of pressure, when rocks are forced by folding to occupy less 

 space. And when from this cause the original distinction between 

 minor layers of rock disappears, and is replaced by new planes of divi- 

 sion, and when the original mineral character of the rock disappears 

 to give rise to a crystalline texture, and to minerals which are never 

 found in the strata, the rocks are said to be metamorphosed. After- 

 wards it may be seen that these changes go so far, that lavas and 

 granites appear to be formed out of sands and mud by the action of 

 the heat to which pressure gives rise. 



All the older Primary rocks of this country, and in other countries 

 the neAver rocks of all f geological ages, have been more or less meta- 

 morphosed ; clays are' thus changed into slates, sandy clays into 

 schists, certain sandstones into quartzites, and ordinary limestones 

 into crystalline or statuary marble. When limestone is thus altered, 

 its texture becomes amorphous or granular from the small size of the 

 calcite crystals of which it consists ; all traces of fossils disappear, fre- 

 quently crystals of garnets, augite, or other minerals form ; and there 

 may be irregular films of colour, or a greyish tinge, in place of the 

 varied or diffused colour which the rock originally possessed. 



Carbonate of lime being easily soluble in heated water, a compara- 

 tively low temperature may have been sufficient to bring about these 

 changes. An example of such a limestone in this country is seen in 





