IGNEOUS FORMATIONS. 295 



In examining the mineralogical composition of the deposits found in 

 different cavities a remarkable uniformity appears, and this is in strong 

 contrast to the formations of the second period. 'The bottom of each mass 

 is quite evenly granular, varying somewhat in density but usually compact, 

 though resolvable with a good lens into a mixture of rude tablets and 

 corresponding prisms which seldom, it' ever, show a crystal termination. 

 These two minerals were found to be, respectively, stilbite and laumontite. 

 It is extremely rare that these two species are found in separate layers. 



While the lower part of each deposit consists of the mixture above 

 described, there may appear minute, isolated spherules of reddish thom- 

 sonite. At first few and far between, these spherules increase steadily in 

 number upward in every deposit, and at last predominate strongly and 

 even make up the entire upper layer. As the thomsonite increases it may 

 frequently be observed that stilbite decreases, SO that in the upper part of 

 the mass it is not rare to find laumontite alone in the interstices between 

 the thomsonite sperules. 



Deposits similar to those just described occur in fissures which are uot 

 uncommon in the upper part of the basalt sheet and which reach a width 

 of 1 or 2 inches. But the similarity is with the upper part of the stratified 

 deposits in the round cavities, i. e., there is a development of thomsonite 

 in a loose aggregate of spherules with laumontite or hole in subordinate 

 quantity, and stilbite is usually wanting. Closely corresponding in compo- 

 sition to the filling of distinct fissures is that occurring in the more or less 

 angular spaces at and near the crust of the sheet and on the actual upper 

 surface of the flow when open spaces existed. As has been pointed out, 

 the structure of the crust is due to the breaking and crumbling of the 

 first-formed shell of a moving- lava stream. 



Genesis of the reddish deposits. — In consideration of all the above facts it seems 

 to the writer that we may safely draw a number of conclusions concerning 

 this earlier zeolitic formation. The appearance of the minerals deposited 

 in stratified masses on the floor of amygdaloidal cavities, and the structure 

 of the deposit itself, indicate that the solution out of which the minerals 

 crystallized had comparatively easy access to the cavities, so that the liquid 

 settled in the lower parts. The rate of crystallization was so rapid that 



