70 TRANSFORMATIONS OF ROCK-MATTER. 



the moment it is laid down as sediment or discharged from 

 the crater of a volcano, "begins to suffer change and transfor- 

 mation. The attraction of cohesion, pressure, the percolation 

 of chemical solutions, heat, magnetic currents, crystallisa- 

 tion, and the like, are all more or less altering its internal 

 texture, and in the long-run its external aspect or structure. 

 In course of time, and under the operation of these forces, 

 the softest mud "becomes compacted into shale, sand into 

 sandstone, gravel into conglomerate, peat-moss into coal, 

 and coral-growths into limestone ; and by a farther trans- 

 formation shales may become glistening clay-slates, sand- 

 stones quartzites, coals anthracites, and limestones sparkling 

 and. crystalline marbles. This kind of transformation, or 

 Metamorplnsm, as it is technically termed, forms one of the 

 most wonderful as it does one of the most difficult chapters 

 in geology, and it is to place it in as simple and intelligi- 

 ble a light as the nature of the subject will permit that we 

 attempt the present Sketch. Of course, it is not to be 

 expected that what is often a matter of doubt and difficulty 

 to the professed geologist can be made easy to the compre- 

 hension of the casual inquirer; but an indication of the 

 processes of metamorphism can be given, and such indi- 

 cation may lead to a more satisfactory conception of the 

 character and formation of rocks in general. 



One of the most obvious agents in the re-formation of 

 disintegrated rock-matter is simple mechanical pressure. 

 As layer after layer of sedimentary matter (mud, clay, sand, 

 gravel, &c.) accumulates in seas and estuaries, the lower and 

 earlier are necessarily pressed upon by all those above them, 

 and thus they are gradually consolidated muds and clays 

 into shales, sands into sandstones, and gravels into conglo- 

 merates. This pressure at great depths must be enormous ; 

 and as it is incessant in its action, we can readily perceive 

 how rock-substances may be altered in their structure and 



