and the allied Rocks. 59 



submarine eruptions. This solution has been shown, by actual 

 experiment, to be a necessary consequence of such action. 

 This fact corresponds most completely with the above deduc- 

 tions. Quartz usually forms the first lining of the geode or 

 amygdaloidal cavity, when it is found at all, and, moreover, 

 it is the most abundant of all amygdaloidal minerals. 



Quartz may also proceed from decompositions of the rock 

 in the cold, and incrustations of this kind are known to occur ; 

 but such an explanation does not account for its generally 

 preceding all other species in filling cavities and seams in trap 

 rocks, and is insufficient to produce the large deposits of silica, 

 sometimes amounting to many tons in a single geode. 



It should not be understood that the quartz is supposed to 

 be derived always from the same heated waters that attended 

 the formation of the containing rock ; for later eruptions in 

 the same region might, at a subsequent period, produce a like 

 result : yet, as its place in the series proves it to be the earliest 

 in formation, it has probably been generally deposited from 

 the water heated during the eruption of the rock. Leaving 

 quartz, we pass to the other minerals. 



It is a striking fact that the minerals next to quartz in the 

 table given — datholite, yrehnite and analcime — contain less 

 water than either of the following species. While the others 

 include from 10 to 20 per cent., the first, datholite, has but 

 5 per cent., prehnite about k\ per cent., and analcime 8 per 

 cent.* This fact certainly leans towards the view of their 

 having originated at a somewhat more elevated temperature 

 than the other species — the same conclusion that is drawn 

 from their lower position in geodes. 



The fact, also, that prehnite has been found forming pseudo- 

 morphs, bears the same way ; for heat would be necessary, in 

 all probability, to aid in removing the original mineral. The 

 vast extent of some prehnite veins — occasionally, as Dr. 

 Jackson has observed, three or four feet wide — refers to an 

 origin like that of the quartz in similar rocks. Indeed, there 

 seems little doubt that prehnite is often derived from that 



* The following table shows the percentage of water, and gives at the 

 same time a general view of the composition of the zeolites: — 



Silica, boracic acid, lime. — Datholite (5 Aq.). 



Silica, alumina, lime. — Prehnite (4j Aq.). Heulandite (14 Aq.). Scolecite 

 (13LAq.). Epistilbite (14 Aq.). Stilbite (17 Aq.). Laumonite(17 Aq.). 



Silica, alumina, lime and potash, or soda. — Mesole (12 Aq.). Thomsonite 

 (13 Aq.). Phillipsite (17Aq.). Chabazite (21 Aq.). 



Silica, alumina, and either soda, baryta or strontia. — Analcime (8 Aq.). 

 Natrolite (9i Aq.). Harmotome (15 Aq.). Brewsterite (1.3 Aq.). 



Silica, lime and potash. — Apophyllite (16 Aq.). 



Silica, lime. — Dysclasite (16£ Aq.). 



