ELEMENTS OF GEOLOGY. 
580^ 
warn the reader that we are here entering upon ground of 
controversy^ and soon reach the limits where positive induc¬ 
tion endsj and beyond which we can only indulge in specula¬ 
tions. It was once a favorite doctrine^ and is still maintain¬ 
ed by many, that these rocks owe their crystalline texture, 
their want of all signs of a mechanical origin, or of fossil con¬ 
tents, to a peculiar and nascent condition of the planet at the 
period of their formation. The arguments in refutation of 
this hypothesis will be more fully considered when I show, in 
Chapter XXXV.jto how many difterent ages the metamor- 
phic formations are referable, and how gneiss, mica-schist, 
clay-slate, and hypogene limestone (that of Carrara, for ex¬ 
ample) have been formed, not only since the first introduc¬ 
tion of organic beings into this planet, but even long after 
many distinct races of plants and animals had flourished and 
passed away in succession. 
The doctrine respecting the crystalline strata implied in 
the name metamorphic may properly be treated of in this 
place; and we must first inquire whether these rocks are 
really entitled to be called stratified in the strict sense of 
having been originally deposited as sediment from water. 
The general adoption by geologists of the term stratified, as 
applied to these rocks, sufficiently attests their division into 
beds very analogous, at least in form, to ordinary fossilifer- 
ous strata. This resemblance is by no means confined to the 
existence in both occasionally of a laminated structure, but 
extends to every kind of arrangement which is compatible 
with the absence of fossils, and of sand, pebbles, ripple-mark, 
and other characters which the metamorphic theory supposes 
to have been obliterated by plutonic action. Thus, for ex¬ 
ample, we behold alike in the crystalline and fossiliferous 
formations an alternation of beds varying greatly in compo¬ 
sition, color, and thickness. We observe, for instance, gneiss 
alternating with layers of black hornblende-schist or of 
green chlorite-schist, or with granular quartz or limestone; 
and the interchange of these different strata may be repeat¬ 
ed for an indefinite number of times. In the like manner, 
mica-schist alternates with chlorite-schist, and with beds of 
pure quartz or of granular limestone. We have already seen 
that, near the immediate contact of granitic veins and vol¬ 
canic dikes, very extraordinary alterations in rocks have 
taken place, more especially in the neighborhood of granite. 
It will be useful here to add other illustrations, showing that 
a texture undistinguishable from that which characterizes 
the more crystalline metamorphic formations has actually 
been superinduced in strata once fossiliferous. 
