On the Minerals of Trap and the allied Rocks. 57 
should therefore have all the necessary ingredients at hand for the 
required 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 conditions at hand 
necessary for producing, by infiltration, the zeolites and the chlo- 
rite nodules of these rocks; the alumina, alkalies and lime, con- 
tribute, 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 farther indicates the pro- 
bable truth of these views ; for it appears evidently to be a precip- 
itate 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 pro- 
ceeded, 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 prin- 
cipal 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 adjoining walls of a seam or dyke— 
which receive the drainings from the rock of the dyke—are often 
penetrated by zeolitic minerals. 
It may be thought that I am giving eine influence to a favor- 
ite theory, and in the minds of some, these conclusions may be 
set down among mere speculations in science. But the circum- 
stances attending submarine igneous action, I am _ persuaded, 
is not generally apprehended. What is the condition of the deep 
* Chlorite consists of the same elements as augite or hornblende, except that the 
lime is excluded and water added. They are silica, alumina, magnesia, oxyd of 
iron, with 12 per cent. of water. . 
Vol. xuix, No. 1.—April-June, 1845. 8 
