EELATIVE EAPIDITY OP WEATHEEINa 259 



the sedimentary rocks (exclusive of the calcareous varieties) are 

 themselves the actual products of these adjustments. The con- 

 glomerates, sandstones, shales, and argillites are but the detrital 

 remains of eruptive rocks which under the various weathering 

 influences have become disintegrated and decomposed, their more 

 soluble constituents quite or in part removed, and the residues 

 laid down and consolidated under conditions such as to-day 

 exist upon or near the surface of the earth. They have, it is 

 true, been laid down under water; they are subaqueous, but 

 their decomposition and disintegration was subaerial. Hence, 

 when elevated above the ocean's level to become a part of the 

 dry land, they are for the most part comparatively stable, sub- 

 ject to only such chemical changes as oxidation, and it may be 

 dehydration. All other things being equal, then, those siliceous 

 rocks which are the product of mechanical sedimentation will be 

 found far less susceptible to the chemical action of the atmos- 

 phere and meteoric waters than are the eruptives. "While they 

 may undergo a transformation into soils, it is mainly through 

 the disintegrating effects of heat and frost. Sedentary soils 

 resulting from such disintegration resemble, therefore, their 

 parent rock more than those of any other class. 



Turning now to calcareous rocks, we shall find a quite differ- 

 ent state of affairs prevailing, owing to the different chemical 

 nature of the material and its ready solubility. These rocks 

 represent, in fact, the soluble portions of the eruptive rocks 

 which have been leached out during the process of decomposi- 

 tion. They are themselves solution products, although their 

 immediate deposition has been brought about through mechanical 

 agencies, as in the laying down of beds of shell marl upon a 

 sea-bottom. The lime leached out of terrestrial rocks is carried 

 in solution into the sea, where, taken up by molluscs and corals 

 as a carbonate, it becomes precipitated to the bottom on their 

 death, and may reappear as a limestone, or, if mixed with suffi- 

 cient quantities of other constituents, as a marl, calcareous 

 sandstone, or shale. Such on their re-elevation are still subject 

 to chemical change, owing to the ready solubility of lime car- 

 bonate in terrestrial waters, and so the endless round begins 

 once more. Reference has already been made to the amounts 

 of lime carbonate that may thus be annually removed from 

 the earth's surface, but one may add here, that, according to 



