PRESSURE AND CHEMICAL ACTION. 71 



texture by this force alone. But during this mechanical 

 pressure chemical actions and reactions are perpetually 

 taking place in all rock-masses, and thus its effects are facili- 

 tated and rendered much more perceptible. A bed of peat, 

 for example, may be solidified by compression, but during 

 its chemical passage into coal it undergoes a process of 

 softening and bituminisation which enables mere pressure 

 to act with greater uniformity and effect. In the earth's 

 crust, therefore, wherever chemical transformations take 

 place, or heat is induced by chemical actions, mechanical 

 pressure will act with increased efficiency ; and as these are 

 almost everywhere present, we may regard the two combined 

 (that is, pressure and chemical action) as amongst the most 

 important agents in the metamorphism or transformation 

 of rock-matters. And those who have witnessed the effects 

 of the hydraulic press the conversion of the softest pulp 

 into a solid mass, and the loosest powder into a hard and 

 brittle stone can have little difficulty in conceiving the 

 myriad-fold greater result of thousands of feet of superin- 

 cumbent strata, and the continuance of their weight and 

 pressure for unknown ages. 



But while all rock-substances are thus continually pressed 

 upon and transformed by those that lie above them, the in- 

 filtration of chemical solutions as well as chemical reactions 

 among the particles of their own mass are also materially 

 assisting in changing their composition and texture. The 

 loose shelly sand of the sea-shore is often cemented into a 

 compact conglomerate* by the percolation of rain-water, 

 which, dissolving the limy matter of the shells, diffuses it 



* This " littoral concrete," as it has been lithologically termed, may be 

 witnessed among the sand-drifts, and along the shores of many parts of 

 the British Islands. Composed of sand, shells, and pebbles, it is often of 

 stony hardness, and might be mistaken for an older rock, were it not for 

 the imbedded shells, which are all recent, and to be found in the neigh- 

 bouring seas. 



