264 Mr J. D. Dana on the Origin of Trap Minerals. 



are found in the felspars ; lime, magnesia, and iron, in augite 

 or hornblende ; iron and magnesia in chrysolite. These are 

 all the constituents needed, except a little baryta for one 

 species^ The felspar decomposes readily and gives up its 

 ingredients, its potash or soda, silica and alumina, The same 

 is true of augite and chrysolite, which afford magnesia, lime, 

 silica, and iron. With water to infiltrate, we should, there- 

 fore, have all the necessary ingredients at hand for the re- 

 quired compounds. The fact already stated, that zeolites have 

 been found as stalactites in caverns, seems to prove that they 

 do actually result from decompositions and recompositions, 

 such as have been supposed. Thus, we have all the condi- 

 tions at hand necessary for producing, by infiltration, the 

 zeolite and the chlorite nodules of these rocks. The alumina, 

 alkalies, and lime, contribute, along with, a portion of the 

 silica, to the zeolites ; and the magnesia, iron, and another 

 portion of the silica, to the chlorite,* often as abundant as 

 the former. The amygdaloidal nodules frequently have a 

 green coating, which further indicate the probable truth of 

 these views ; for it appears evidently to be a precipitate from 

 the solution before a crystallization of the zeolites took place 

 — a settling, perhaps, of the insoluble impurities taken up by 

 the filtrating fluid in its passage through the rock, or of the 

 formed chlorite, less soluble than the zeolites. Occasionally, 

 when the rock contains copper, these nodules have an earthy 

 coating of green carbonate of copper — -the carbonate having 

 proceeded, apparently, from the native copper of the rock, by 

 the same process as explained. 



The hypothesis of filtration seems, then, to be at least the 

 principal source of these minerals. In some instances the 

 filtrating fluid may have derived its ingredients from distant 

 sources. The salts of sea-water may act an important part 

 in these changes. Silica is dissolved on a grand scale dur- 

 ing submarine eruptions, as we have elsewhere urged, and 

 is thence distributed to the rocks around. Lime, also, is 



^ Chlorite consists of the same elements as augite or hornblende, except that 

 the liine is excluded and water added. Thoj^ are, silicu, alumina, magnesia, 

 oxide of iron^ with 12 per cent, of water. 



