60 On the Minerals of Trap and the allied Rocks. 
mation an elevated temperature, they are those first deposited— 
the first in the above series. A few considerations will place this 
if possible in a clearer light. 
Quartz, as we have stated in a preceding page, and fully re- 
marked upon elsewhere, enters largely into solution during sub- 
marine eruptions. This solution has been shown, by actual ex- 
periment, to be a necessary consequence of such action. ‘This 
fact corresponds most completely with the above deductions, 
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 du- 
ring 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 ta- 
ble 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 44 per cent., and analcime 8 per cent.* This 
fact certainly leans towards the view of their having origi- 
* The following table shows the percentage of mais and gives at the same 
time a general view of the composition of ae olites 
pom boracic acid, lime.—Datholite (6 A 
umina, lime.—Prehnite (44 aay “Hevlandite (14 nie ) Scolecite (135 
Aq. . Epistilbite “ Aq.) Stilbite (17 Aq.) ro aay (17 
Silica, alumina, lime and potash or soda.—Mesole (12 i Thomsoniie as 
Aq ) Phillipsite (17 Aq.) Chabazite (21 Aq.) 
ica, alumina, and either soda, baryta or strontia.—Analcim (8 Aq.) Natrolite 
(94 Aq.) Harmotome (15 Aq.) Brewsterite (13 Aq.) 
Silica, lime and potash.—Apophyllite (16 Aq.) 
Silica, lime.—Dysclasite (163 Aq.) 
