THE ORIGIN AND DESCENT OF ROCKS. 43^ 



aid each other. The ordinary pressures arise from the weight of the 

 overlying material, and these of course increase with depth. Extraor- 

 dinary pressures arise from the shrinkage of the earth and perhaps 

 from other sources. The fragments of the clastic material, on being 

 pressed together for long periods, weld more or less at the points of con- 

 tact. If they are irregular, angular, or elongate, they come to inter- 

 lock more or less like the fragments of macadam, and this cooperates 

 with the welding. The process is greatly aided by water-bearing solu- 

 tions of lime, silica, etc. which are deposited at the points where the 

 fragments press upon each other. It is here that the capillary spaces 

 are most minute and deposition is most liable to take place. Some- 

 times a film of mineral matter is laid down over the surfaces of the 

 fragments and serves to bind them together. This process goes on 

 wherever the ground- waters are in a depositing condition, just as the 

 opposite process of disintegration takes place wherever the waters are 

 in a solvent state. At and near the surface of the land, the waters are 

 usually in the latter condition and disintegration is in progress, as already 

 noted, but this is not always so. At times and places, the w^ater from 

 within the rock-mass may come to the surface and evaporate, and in so 

 doing leave all its dissolved material on the surface, or within the outer 

 pores of the mass, as cementing material. The exterior thus becomes 

 firmly bound together, ^^case-hardened," as it is termed. This may 

 be seen in the drying of a lump of mud, the exterior of which often 

 becomes quite firm. It is seen in quarry-rock, especially sandstone, 

 which is sometimes soft and easily worked when taken w^et from the 

 earth, but which hardens as the water — the ''sap" of the quarrymen — 

 dries out and deposits its solutes in the capillary spaces of the grains 

 of the surface. It is obvious that it is the very last of the "sap" which 

 contains the most concentrated^ solutes, and that this last remnant is 

 held in the minute capillary spaces where the grains touch each other, 

 and hence the last stage of drying leaves the cement at the points where 

 it is most effective. In natural exposures of sandstone, the pores of the 

 outer shell sometimes, become almost completely filled in this way with 

 sihcious deposits, and the sandstone is changed into a quartzite. 



In the sea, and in the deep water underground, the common habit of 

 the water is to deposit more than to dissolve, though it is doing more 

 or less of both. As a rule, therefore, loose material in these situations 

 becomes bound more or less firmly into rock, and hence what were origi- 



