THE MAJOR SOIL-FORMING MINERALS. 39 



expected ; thus we find not unfrequently that certain sandstones are 

 materially softened, and their resistance destroyed, by treatment with 

 even moderately dilute acid, while silica and the usual zeolite bases pass 

 into solution. It is not often, however, that zeolitic material alone 

 cements the sandstone ; it is most frequently associated with siliceous, 

 calcareous and sometimes even with ferruginous cementing material. 1 



CALCITE AND LIMESTONES. 



Calcite or calcareous spar is one of the minerals most com- 

 monly known in the crystallized form, and is readily recognized 

 by its perfect cleavage in three directions, producing cleavage 

 forms with smooth, rhomb-shaped faces (rhombohedrons) ; 

 these are sometimes colorless and perfectly transparent, and 

 laid on printed paper show the letters double. But it may be 

 whitish-opaque, and of various colors, which may also be im- 

 parted to the limestones formed from it. It is readily dis- 

 tinguished from quartz, which it sometimes resembles, by its 

 cleavage, its inferior hardness, being easily scratched with a 

 knife; and by its effervescence with acids, the latter being the 

 crucial test when other marks are unavailable, as when it forms 

 soft granular masses or " marls." In all cases it can be recog- 

 nized by its crystalline form under the microscope, even when 

 the substance containing it has been pulverized in a mortar. 

 The great importance of this compound calcic carbonate 

 from the agricultural point of view renders it desirable that 

 it, as well as limestones as such, should be recognized, when 

 seen, by every farmer. 



In mass the pure mineral constitutes white marble ; colored or vari- 

 egated marbles are more or less impure from the presence of other 

 minerals. Some compact limestones also are nearly pure ; and as sup- 

 plying only a single ingredient of plant food these would not be much 

 better soil-formers than quartz or serpentine. But it is quite otherwise 

 with common limestones ; the mass of which, it is true, is formed of 

 calc-spar, but owing to its origin, is in the great majority of cases so far 

 commingled with other matters of various character, that limestones are 



1 A zeolitic mass, at first gelatinous and then becoming granular-crystalline is 

 frequently observed oozing from the lower surface of newly made concrete reservoir 

 dams: just as we find similar oozes consolidated into natrolite crusts in tin 

 crevices of natural sandstones. 



