80 Intelligence and Miscellaneous Articles. 



chemical and geological science. We are probably acquainted with 

 no rocks not of aqueous origin ; and the intrusive form so often 

 assumed by granites, syenites, dolerites, and even by ophiolites and 

 limestones, is not essential but accidental. 



In my report for 1856, p. 485, I have insisted that the separation 

 of oxide of iron from certain strata, and its accumulation in others, is 

 to be ascribed to the reducing and solvent action of organic matters. 

 This is exemplified in the fire-clays and iron-stones of the coal for- 

 mation, as well as in the series of the Hudson-river group described 

 in my report, in the fire-clays and greensands of the cretaceous for- 

 mation of New Jersey, and many other instances. It is by the 

 alteration of such materials that the white felspathic rocks of 

 metamorphic regions have been formed ; and it is probable that beds 

 of iron ore always owe their origin to the intervention of organic 

 matters, so that the presence of such ores, not less than that of 

 graphite, points to the existence of organic life even during the 

 Laurentian or so-called Azoic period. 



The waters which dissolve out the oxide of iron from sediments, 

 also remove lime and magnesia, especially if these bases are present 

 in the condition of carbonates, and hence these bases, especially the 

 latter and more soluble, are generally absent from white clays and 

 felspar rocks. If, however, the quantity of lime be large as com- 

 pared with the iron, this may be removed while a portion of lime 

 remains ; if, on the other hand, the reducing agency of organic 

 matters be excluded, carbonated waters may remove lime and mag- 

 nesia, leaving the peroxide of iron behind. 



From mj'- own and others analyses of the alkaline mineral waters 

 derived from argillaceous rocks, it will be seen that the salts of 

 potash in these waters are generally in very small quantity when 

 compared with the salts of soda, although potash predominates in 

 argillaceous shales and clay-slates. The soda is therefore gradually 

 removed from these rocks by infiltrating waters, while the potash 

 remains behind, and hence it happens that when these rocks, from 

 which the lime, magnesia, and oxide of iron have been dissolved, are 

 subjected to the process of metamorphism, potash-felspar will pre- 

 dominate, together with quartz, from the deficiency of bases, while 

 silicates like cyanite and staurotide may be formed from the excess 

 of alumina. The more quartzose sediments, other things being equal, 

 are most permeable to water, and hence will have lost greater pro- 

 portions of soda, lime, magnesia, &c., than the finer clays and marls. 



I have here indicated a few principles which must I think for the 

 future serve as guides in investigating the chemistry of rocks, whe- 

 ther stratified or j)lutonic. You will see how the action of these 

 laws necessarily divides the silico- aluminous sedimentary rocks into 

 the two great classes recognized by Bunsen and Durocher in their 

 investigations of igneous rocks. In the trachytic and granitic divi- 

 sion the silica and potash predominate, and the soda, lime, mag- 

 nesia and iron are present only in small quantities, while in the 

 pyroxenic rocks silica and potash are less abundant, and soda-fel- 

 spars with more or less liasic silicates, of lime, magnesia, and oxide 

 of iron predominate. — Silliman's American Journal for May 1858. 



