142 On Certain Cavities in Qfiiartz, S)'c. 



projecting points of the cavity. The action of water, therefore, 

 is capable of reducing quartz to a fluid and plastic state. 



The occurrence of calcareous spar is most frequent in veins, 

 cavities, or fissures, associated with quartz and other minerals. 

 Its formation is likewise dependent upon the filtration of water, 

 holding carbonate of lime in solution, which assumes its crystal- 

 line character from the slow percolation or evaporation of the 

 water. Water, therefore, saturated with siliceous particles, in its 

 progress through the earth, will get access to cavities lined with 

 calcareous spar, and here gradually precipitate them. These par- 

 ticles will regularly accmnurate, and as the water which conveyed 

 them there filters out, they will condense more and more, and ulti- 

 mately consolidate around the projections and within the angular 

 sinuosities of the uneven surface of this mass of aggregated crys- 

 tals. The quartz, thus becoming plastic, will mould itself to the 

 spar, and this afterwards becoming dissipated by agents^incapable 

 of acting on the quartz, will leave its impressions accm'ately de- 

 fined, and communicate to these specimens their peculiar char- 

 acter. 



The question regarding the separation of the carbonate of lime 

 from the quartz next arises ; and the agent most likely to accom- 

 plish this without impairing the integrity of the quartz or forming 

 a new and insoluble compound in the cavities, I conceive is water. 

 Although water itself is not a good solvent of carbonate of lime, 

 yet, when charged with carbonic acid, its power is much increased. 

 Now, it is well known that there are many local sources of car- 

 bonic acid gas, and even in our own county, it is frequently 

 found collected in large quantities in the bottoms of wells. That 

 this gas, always generating by some subterranean process, com- 

 bines readily with water in its vicinity, is evidenced by the fre- 

 quent occurrence of carbonated springs : and as the absorbent 

 power of the water is increased in a direct ratio with the pressure, 

 so is its solvent power augmented in proportion to the accumula- 

 tion of carbonic acid. Carbonic acid gas, therefore, confined 

 within cavities beneath the surface of the earth, must necessarily 

 be exposed to considerable pressure, and under these circumstances 

 will be copiously absorbed by water in contact with it. Water, 

 thus impregnated, being conveyed to the mass of crystals im- 

 bedded within the quartz, will effectually dissolve it and wash it 

 out from the cavities it formed, without in the least affecting the 

 conformation of the quartz. 



