METAMORPHISM BY PRESSURE. 357 



6. The sublimation of some portion of the neighbouring substances. 

 Thus the charring of coal, the expulsion of sulphur and bitumen from 

 shale, are directly connected with the heating power of the igneous 

 rock. It is probable that some peculiar conditions were required for 

 such effects of contact metamorphism in submarine depths, where most 

 of these operations were performed. 



Metamorphism of Rocks. 



Structural Metamorphism. In daily experience, we see some 

 degree of consolidation effected in calcareous deposits by the concre- 

 tionary or crystalline coherence of the particles. But we scarcely 

 perceive any induration of clay or agglutination of sandstone without 

 infiltration of salts, enormous pressure, or the application of heat. By 

 the subsidence of the strata to some thousands of feet or yards, which 

 has unquestionably happened in very many cases, these favourable 

 influences were brought into action. The oldest strata were upon the 

 whole sunk to the greatest depth, and in consequence have experienced 

 the greatest amount of pressure and heat, and these are on the whole 

 the most consolidated; the clays have become slate and the sands 

 quartzite. 



Effects of Pressure. The lowest strata of the coal basin of South 

 Wales, which were deposited nearly at the sea-level, were necessarily 

 sunk during the latest palaeozoic periods about 12,000 feet below the 

 surface. In these a partial slaty cleavage appears. The Old Red 

 Sandstone strata, several thousand feet thick, which were still deeper 

 and more heated, are more marked by cleavage ; and the Silurian and 

 Cambrian, still deeper by thousands of feet, are even more distin- 

 guished by that structure. It is chiefly in what were the deeper 

 parts of the basin that this effect occurs ; for on the north side of 

 the South Wales coal-field the Silurian strata and the Old Red 

 Sandstone are often free from cleavage, and cleavage is there only 

 partially exhibited in the Cambrian. 



Farther to the north, as in the district of the Malvern Hills, 

 Woolhope, Abberley, Dudley, the Silurian rocks and all above them 

 are free from cleavage, or exhibit it only in a very slight degree, and 

 along some small and limited spaces. Yet these districts are marked 

 by great and violent flexures, and even reversals of the strata, so that 

 pressure seems sometimes to have failed entirely in producing cleavage. 

 This is the more curious, because, in the same country, parallel to the 

 once heated basalt dyke of Brockhill, the Old Red Sandstone shales 

 have developed a rude cleavage. In these districts there is no reason 

 to admit more than 5000 to 8000 feet of depression. 



The Cambrian rocks of Charnwood Forest and the base of Ingle- 

 borough are full of cleavage, crossing great curvatures of the strata. 

 Those curvatures preceded the formation of the Old Red Sandstone. 

 There is no cleavage in any of the upper or middle palaeozoic strata, 

 which, in the utmost depression which we can trace, may have been 

 sunk some 5000 or 6000 feet deep in Yorkshire, and some 8000 feet 



