56 J. D. Dana on the Minerals of Trap 



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 therefore have all 

 the necessary ingredients at hand for the required compounds. 

 The fact already stated, that zeolites have been found as sta- 

 lactites in caverns, seems to prove that they do actually re- 

 sult from decompositions and recompositions, such as have 

 been supposed. Thus we have all the conditions at hand ne- 

 cessary for producing, by infiltration, the zeolites 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 amyg- 

 daloidal nodules frequently have a green coating, which fur- 

 ther indicates the probable truth of these views ; for it appears 

 evidently to be a precipitate from the solution before a cry- 

 stallization 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 so- 

 luble than the zeolites. Occasionally, when the rock contains 

 copper, these nodules have an earthy coating of green carbo- 

 nate 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 during 

 submarine eruptions, as we have elsewhere urged, and is 

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

 in a similar manner. But the rock itself has often afforded 

 the ingredients for the forming minerals, during the passage 

 of the filtrating fluid through it. By the same means, the ad- 

 joining walls of a seam or dyke — which received the drainings 

 from the rock of the dyke — are often penetrated by zeolitic 

 minerals. 



It may be thought that I am giving undue influence to a 

 favourite theory, and in the minds of some, these conclusions 

 may be set down among mere speculations in science. But 



* Chlorite consists of the same elements as augite or hornblende, ex- 

 cept that the lime is excluded and water added. They are silica, alumina, 

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



