American Scientific Association, 295 



more soluble bicarbonate of magnesia, which by further evapor- 

 ation is separated as hydrous carbonate, either alone or mingled 

 with carbonate of lime. When these magnesian precipitates 

 are gently heated under pressure they are changed into magne- 

 site or dolomite. Thus are explained the magnesian rocks 

 associated with gypsums and with rock salt. The action of solutions 

 of bicarbonate of soda may in like manner separate the lime from 

 sea-water and give rise to solution of bicarbonate of magnesia ; 

 in this way are formed the magnesian limestones which are not 

 associated with gypsum. The intervention in this process of 

 the waters of alkaline metalliferous springs will explain the 

 metalliferous character of many magnesian rocks. The source of 

 the bicarbonate of soda has been the decomposition of felds - 

 pathic rocks to form clays and clay slates. The action of this 

 alkaline carbonate upon the lime and magnesia salts of the primi- 

 tive sea has been the source of limestone and dolomites, as well 

 as of the sea salt which we find in the ocean, at the same time 

 that the intervention of the carbonic acid of the atmosphere 

 which has been through the medium of the soda, fixed in the 

 form of carbonate of lime, has served to purify the air and fit 

 it for the support of higher orders of plants and animals. In 

 this relation between the atmosphere, the argillaceous rocks, 

 the limestones and the salt of the sea, we have a remarkable 

 illustration of the balance of chemical forces in inorganic nature. 



FORMATION OF SILICIOUS ROCKS, 



Mr. Sterry Hunt then spoke of sediments resulting from the 

 disintegration and chemical decomposition of quartzose, feldspathie 

 and pyroxenic rocks. In these the coarser portions consist of 

 quartz and of feldspar containing potash, while the finer clays haw 

 less silica but more alumina, and besides alkalies lime, magnesia 

 and iron, which are rare in the coarser sediments. These latter 

 being more pervious to water, the small portions of soda, lime 

 and magnesia still remaining are removed by lixiviation, while 

 the clays retain these bases. When these different sediments 

 are altered and crystallized we shall have on the one hand 

 granitic or trachytic, and on the other pyroxenic rocks, the 

 two great types recognized in igneous rocks, all of which Mr. 

 regards as derived from the alteration and fusion of sedimentary 

 strata. To the gases and vapors evolved by the fusion of deeply 

 buried strata are to be referred the phenomena of earthquakes 



