162 



The Naturalist, 



Bavarian ruby. Its colour is attributed to the presence of the oxides 

 of titanium and manganese. Common quartz is crystalline or 

 massive, white, or grey ; also red or brown, and is a very common 

 constituent of rocks. Chalcedonies comprise the varieties of quartz 

 which possess a scaly fracture, and are susceptible of a fine polish ; 

 flints, on the other hand, have a smooth, even fracture, dull colour, 

 and cannot readily take a polish. Chalcedony is white, grey, blue, 

 green, yellow, or brown, and occurs in stalactitic reniform or botryoidal 

 masses. Heliotrope or bloodstone is a dark green variety with red 

 spots of jasper. Agates are mixtures of chalcedony in layers with 

 jasper, amethyst, or common quartz, and abound in the amygdaloids 

 of our own and other countries. 



Origin. Reverting to the primitive history of our globe, when the 

 solid materials of the earth were in a state of igneo-aqueous fusion, 

 surrounded by a dense atmosphere of carbonic, sulphuric, and hydo- 

 chloric acid gasses, it is supposed by Dr. Ste:ry Hunt that 

 subsequent upon a certain amount of cooling, an acid rain would fall 

 upon the fluid mass, and decomposing the silicates of potash, soda, 

 lime, magnesia, iron, &c., the carbonic, sulphuric, and hydrochloric 

 acids would unite with the liberated bases, and silica being set free, 

 would precipitate in the form of massive or crystalline quartz. 



With reference to the occurrence of quartz in fissures, we cannot 

 doubt that ancient, like modern veins, have been channels for the 

 discharge of subterranean mineral waters, containing silica in solution, 

 which, rising towards the surface, would deposit crystals of quartz in 

 the sides of the fissures, as a result of the deduction of solvent power 

 consequent upon a diminution of the pressure as the waters rise 

 nearer the surface. The waters of our river- springs, &c., as pre- 

 viously remarked, are more or less charged with silica in solution, 

 derived from the weathering and disintegration of felspathic and 

 other siliceous rocks, the silica of which being separated by chemical 

 agencies, appears in sedimentarv deposits, forming concretions, and 

 even beds of flint, chert, and jasper, as well as various crystalline 

 sandstone. These crystalline sands are, according to Daubree, met 

 with in beds in the sandstone of the Vosges, the Trias, and Permian, 

 and in the tertiary of the Paris basin and elsewhere. All quartzose 

 conglomerates, and the majority of sandstones are undoubtedly 

 mechanical deposits from the ruins of pre-existing quartzose and 

 granite rocks. Vast beds ef quartz also have an organic origin 

 being derived from the spiculie of sponges as well as from silicieous 

 diatoms. 



