218 T. S. Hunt on the Chemical and Mineralogical 
the air of the carbonates of lime and magnesia precepitated, 
and of the amount of chlorid of sodium added to the waters 
iron; leavin 
the elements of granite, trachyte, gneiss, and mica schist, On 
the other hand, the finer marls and clays, resisting the penetra- 
tion of Bevo will retain all their soda, lime, magnesia and oxy 
of iron; and, c containing an excess of alumina with a small 
amount of rellten will, by their metamorphism, give rise to basi¢ 
of the chemical and mechanical causes which we have t 
naturally divide es all the crystalline silico-aluminous roe: 
the earth’s crust into two types. These correspond to the two 
classes of igneous rocks, distinguished first by Professor bier = 
and subsequently by Durocher and by Bunsen, as deri 
two distinct magmas; which these geologists pede 
beneath the solid crust, and which the latter denominates the 
trachytic and pyroxenic types. I have, however, elsewhere en 
deavored to show that all intrusive or exotic rocks are probably 
toting more Binis altered and displaced sediments, and have 
us their source within the lower portions of the stratified crust, 
not i binoat 
It may be ‘wall in this place to cared cH oe observations 
the chemical conditions of rock metam — is 
widest sense the view of Hutton and of hie ae all t the 
talline eat rocks have been produced by the a stration of ‘ 
ou 
4 
: 
i 
a 
into definite mineral species has been effected in two ways: 
by molecular changes, that is to say, by crystallization, and ate 
arrangement of the particles; and, secondly, by chemical reac 
tions between the elements of the sediments. Pseudomor- 
hism, which is the change of one mineral species into anowber 
y the introduction, or the elimination, of some element OF ele- 
ments, presupposes metamorphism ; since only pps rl 
Species can be the ee of this process. 
