DESCRIPTIONS OF ROCKS. 419 
Eiironidy, which were made by steam, or vapor of some kind, 
and are now occupied by minerals. This hydrous or chloritie 
condition is due to alteration, and hence such rocks are 
properly only varieties of the anhydrous instead of being 
distinct kinds. 
The change was probably occasioned by subterranean wa- 
ters, such as exist as streams among the earth’s strata, that 
were encountered by the liquid rock when on its way up 2 
fissure toward the surface. Hydrostatic pressure prevented 
the waters from being driven back by the heat, and conse- 
quently the vapors were forced to penetrate the igneous 
mass. In the region of New Haven, Conn.—lying ‘at the 
south extremity “of the Connecticut Valley—the ‘Triassic 
trap-dikes of the western border of the region, and those 
outside of the Trias, east or west, in the metamorphic 
rocks. are anhydrous, while those in the middle of the valley 
and east of this are mostly hydrous, showing a difference in 
exposure to the waters according to the seographical position 
of the dikes in the valley. Of two parallel ranges of dikes, 
not half a mile apart, and following concentric curves in 
their courses (situated twenty miles and more north of New 
Haven), one (as Percival recognized) is amygdaloidal and 
hydrous, and the other nearly anhydrous; and the positions 
of the two kinds, there and elsewhere in the Connecticut 
Valley, indicate a ‘general relation between the direction of 
the present valleys and that of the subterranean water-chan- 
nels of Mesozoic tame. , 
In very many places coal-like “ inspissated bitumen” 
occurs in the amygdaloidal cavities, which was apparently 
derived from mineral oil that the action of the heat on the 
Triassic carbonaceous shales (in some places abounding in 
fossil fishes) had caused to rise in vapors and penetrate the 
melted rock. ‘he carbonic acid of the calcite that so often 
constitutes the amygdules probably came from the action 
of the heat on limestone encountered at the same time. 
The deoxidizing action of the carbohydrogen vapors is sup- 
posed by J. Lawrence Smith to account for the metallic 
iron found in some trap or doleryte. The minerals which 
constitute the amygdules (see p. 297) are largely such as 
may have been maie by the aid of heat and moisture out of 
the minerals of the rock itself at the points where they occur. 
The water that caused the change could not have come from above 
after the rock was cooled; for the slight surface decomposition the 
