ei x a ; -* ee: bs 
Modern Igneous Rocks and the Primary Pamedlins. 117 
tion in the vicinity, they may have been buried and permeated 
anew with hot siliceous waters, which thus gave rise to the amyg- 
daloidal minerals. Some of these minerals are believed to be 
formed by the percolation of cold water through the rock, pro- 
ducing slow decompositions and forming new compounds. It 
should however be remarked that basalt or trap while still sub- 
merged undergoes but little change from cold water, except from 
abrasion. It is not until exposed to the atmosphere as well as 
moisture that they suffer much alteration or degradation from the 
processes of decomposition. 
It is now very generally admitted that these amygdaloidal min- 
erals are not of igneous origin cotemporaneous with that of the 
rock, and therefore no labored proof is required in this place. It 
is sufficient proof of this, that the cavities, which are inflations 
from steam or gas, must have been made before they could have 
been filled. 
As has been stated, we suppose these trap minerals to be formed 
from the rock that contains them, while the hot waters are pen- 
etrating, and not to be taken or compounded from its surface ; 
neither were they contained in the external currents of heated 
siliceous waters. .'The same minerals sometimes fill cavities in 
the adjoining sandstone, but to so short a distance from the trap, 
that we must believe them deposited from the waters that ex- 
uded from the sides of the dyke, and not from external currents. 
Silica once received into solution will be held far below the 
temperature necessary for dissolving it. We might expect there- 
fore various changes from the introduction of silica, where no 
evidence of heat can be detected. Some siliceous limestones 
may have been thus formed, without a crystallization of the 
lime; and cavities like those in the calcareous sandstone of cen- 
tral New York may have been filled in the same manner with 
quartz crystals, without any evidence of concomitant igneous 
action. — a 
I do not claim that heated waters are the only means by which 
sedimentary rocks have been supplied with silica. The discov- 
eries of fossil animalcules have opened a new source of silex, and 
this, as suggested, may possibly have been the origin of the flint 
in chalk. 
_ Qn the principles explained we may account for the metamor- 
phic porphyries and basalts of the Andes. They lie in a region 
