ean Ei 
Modern Igneous Rocks and the Primary Formations. 121 
of much interest that rocks known to be igneous appear to con- 
tain no hydrated minerals except such as we may believe to ha 
been introduced since their formation, = =~ “ae 
The origin of the zeolites and associated species has been 
shown to be subsequent to the ejection of the rocks containing 
them: that the same is true of the metallic salts is not doubted. 
Talc and chlorite may be suggested as exceptions; but there is 
reason to believe these minerals metamorphic. Von Buch sug- 
gested some years since that tale was mica altered by magnesian 
vapors; and in his account of the rocks of Norway and Lapland, 
as quoted by Lyell, gives several instances of the passage of mica 
slate into magnesian or talcose slate, and supposed to have been 
produced by the change of mica into talc. It is an interesting 
fact that chlorite is common in amygdaloids, filling cavities, 
where like other trap minerals it was deposited after the cavities 
were made, that is, after the ejection of the amygdaloid: and 
like zeolites it may be set down as one of the products resulting 
from the action of heated waters on the containing rock. It is 
therefore probable that in the granitic series it had the same ori- 
gin. ‘The chlorophyllite of Dr. Jackson, or hydrous iolite, is ano- 
ther hydrated mineral in the ancient rocks. Here the evidence, 
derived from its frequent association with iolite, is quite satisfac- 
tory that it has resulted from the alteration or the hydration of © 
® 
this mineral. 
It has been argued that the water in hydrous minerals may be 
retained, under heavy pressure, in the same manner as carbonic 
acid is retained. But the analogy is false. Hot waters will not 
combine with carbonic acid; they even dissolve less than cold 
water. But water will combine with water or aqueous vapor 
The minerals in serpentine are mostly hydrous, and thus sup- 
port the view of its hydro-metamorphic origin. These are Schil- 
ler spar, hydrate of magnesia, talc, nemalite, kerolite, Clintonite, 
&c. As both hydrous and anhydrous crystals are formed from 
aqueous solution we could not expect that all should be hy- 
drous. 
Vol. xiv, No. 1.—April-June, 1843. 16 
