Siluria—Present State of Geology. 315 
and we shall find that the strata are arranged in a definite 
order. 
That the whole crust of our earth has been undergoing a 
metamorphic change from the beginning of time, there 
appears to be little doubt; but the cause of its apparent 
more or less activity, as observed by the geologist in the 
structure of rocks of almost every line of country over 
which he travels, is still a mystery, and can only be settled 
by a close chemical and microscopical examination of the 
rocks. 
The relations of the sedimentary and Plutonian rocks 
are far from being properly understood. The general re- 
ceived notion that the Plutonic rocks, in a state of igneous 
fusion, were impelled from below against the strata that 
contorted them, fractured them, and filled up the spaces 
which they now occupy, appears not to be invariably correct, 
because we frequently find great tracts of country traversed 
in every direction by Plutonic agents without any change 
whatever being produced. ‘‘ Let it, however, be understood,”’ 
says Sir Roderick Murchison, “ that the prodigious extent to 
which the metamorphism of the original strata has been 
carried in mountain chains, and at different periods through 
all formations, though often probably connected with such 
igneous outbursts, must have resulted from a far mightier 
agency than that which was productive of the mere eruption 
of molten matter or igneous rocks. The latter are, in fact, 
but partial excrescences in the vast spread of the stratified 
erystalline rocks—symptoms only of the grand changes which 
resulted from deep-seated causes, probably from the com- 
bination of heat, steam, and electricity, acting together with 
an intensity very powerful in former periods.’ * 
_ Weare still in want of correct information in regard to 
the origin of stratification, lamination, different kinds of 
structure, seams, joints, cleavage planes, and metamorphic 
action; we do not know if these changes are to be ascribed 
either to aqueous, igneous, or electric agencies, or molecular 
_ attraction, as connected with concretionary structure. 
_——- ——_—- ---~ 
* Murchison’s Siluria, p. 4. 
