Mr J. D. Dana on the Origin of Trap Minerals. 201 



be afterwards filled up with other minerals, has been deemed 

 a mystery in science, but the possibility of it is now not 

 doubted. Even flint and agate, as MacCulloch states, are 

 known to give passage to oil and sulphuric acid ; and much 

 more will this take place in the moist rocks before the agate 

 has been hardened by exposure to the air. Silica remains in 

 a gelatinous state for a long period 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 another portion of 

 fluid ; for the process of crystallization having commenced, a 

 new portion of the material is constantly drawn into the 

 same fluid, and the necessary chemical changes are also pro- 

 moted by the inductive influence of the changes in progress 

 — the catalytic action as it is called — one of the most effi- 

 cient, and at the same time one of the most universal, agen- 

 cies in nature. 



Other evidence with reference to amygdaloidal minerals is 

 presented 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 cool- 

 ing rock, and at other times they were of more recent origin. 

 In either case the minerals filling these seams must be sub- 

 sequent in formation to the origin of the rock itself, and could 

 not have proceeded from vapours attending the eruption. 

 These seams sometimes open upward, 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 

 crystallized, 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 Pacific islands ; and Dr Thomson has described and ana- 



