54 J. D. Dana on the Minerals of Trap 



Any deduction, therefore, respecting quartz, holds equally for 

 the associated minerals. 



How a cavity coated with a deposit of chalcedony can still 

 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 hard- 

 ened 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 ca- 

 vity, should yield place to another 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 induc- 

 tive 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 

 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 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 vapours attending the eruption. These 

 seams sometimes open upward and can be seen to have no 

 connexion with the parts below, the rock in this portion being 

 solid. Origin from above or from either side, is the only sup- 

 position 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 

 analysed one (Antrimolite) from Antrim in Ireland near the 

 Giant's Causeway. 



These facts favour throughout the view we urge, that the 

 amygdaloidal minerals have in general resulted from infiltra- 

 tion, and were not necessarily formed simultaneously with the 

 erupted rock. 



