268 Mr J. D. Dana on the Origin of Trap Miner ah. 



in trap rocks, and is insufficient to produce the large deposites 

 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 attend- 

 ed 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 deposit- 

 ed 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, Prehnite, 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 4J 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 pseu- 

 domorphs, 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 

 portion of the silica in solution which entered into combina- 

 tions at the time with the alumina and lime which the sili- 



* The following table shews the per-centage of water, and gives at the same 

 time a general view of the couaposition of the zeolites. 



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



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

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



Silica, alumina, lime, and potash or soda. — Mesole (12 Aq.) Thomsonite (13 

 Aq.) Phillipsite(17 Aq.) Thabazito (21 Aq.) 



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

 trolite (9^ Aq.) Harmotome (15 Aq.) Brewsterite (13 Aq.) 



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



Silica, lime. — Dygclasite (16^ Aq.) 



