On the Minerals of Trap and the allied Rocks. 55 
after deposition, and in this condition is readily permeable by 
solutions. It is not necessary that the fluid which has acted the 
part of a solvent and filled the cavity, should yield place to an- 
other portion of fluid; for the process of crystallization having 
commenced, a new portion of the material is constantly drawn in 
to the same fluid, and the necessary chemical changes are also 
promoted by the inductive influence of the changes in progress 
—the catalytic action as it is called—one of the most efficient, 
and at the same time one of the most universal agencies in nature. 
Other evidence with reference to amygdaloidal minerals is pre- 
sented by the zeolites themselves. 
3. The zeolites occupy veins or seams as well as cavities. Often 
the seams were opened by the contraction of the cooling rock, and 
at other times they were of more recent origin. In either case the 
minerals filling these seams must be subsequent in formation to 
the origin of the rock itself, and could not have proceeded from 
vapors attending the eruption. ‘These seams sometimes open up- 
_ ward and can be seen to have no connection with the parts below, 
the rock in this portion being solid. Origin from above or from 
either side, is the only supposition in such cases. 
Messrs. Jackson and Alger, in their valuable memoir on the 
geology of Nova Scotia, mention the occurrence of crystals of 
analcime attached to the extremity of a filament of copper, the 
copper having been the nucleus about which the solution crystal- 
lized, and state that their formation must have been subsequent 
to the formation of the rock. 
4. Zeolites, moreover, have been found forming stalactites in 
basaltic caverns, as was observed by the writer in some of the Pa- 
cific islands; and Dr. Thomson has described and analyzed one 
(Antrimolite) from Antrim in Ireland near the Giant’s Causeway. 
These facts favor throughout the view we urge, that the amyg- 
daloidal minerals have in general resulted from infiltration, and 
were not necessarily formed simultaneously with the erupted rock. 
5. We remark farther, that no lavas have ever been shown to 
contain at the time of ejection, any of the zeolitic minerals. 
The zeolites of Vesuvius are known to occur only in the older 
lavas, and afford no evidence against our position. The cavities 
in lavas, as far as observed, are empty as they come from the vol- 
canic fires, with the exception of those containing sparingly some 
metallic ores which are condensed within them. Considering 
