308 DYNAMICAL GEOLOGY. ° [Book IL 
regarded as an identity of structure, and the idea has found supporters © 
that the schistose rocks have been in a condition similar to or 
identical with that of many volcanic masses and have acquired their 
peculiar fissility by differential movements within the viscous or 
pasty magma, the solidified minerals being drawn out into layers in 
the direction of motion. Daubrée, availing himself of the researches 
of Tresca on the flow of solids (postea, p. 313), has endeavoured to 
imitate artificially some of the phenomena of foliation by exposing 
clay and other substances to great but unequal pressure.’ It is 
inconceivable, however, that such changes could have been produced 
over the vast areas occupied by foliated rocks. At the same time, 
the intense corrugation and crumpling of these rocks deserves in 
this connection attentive consideration. 
A relation can, indeed, be commonly traced between the complete- 
ness of the crystalline schistose structure and the extent of the corru- 
gation, the most highly puckered masses being also as a rule the 
most crystalline. So universal is this relation as to show that — 
crumpling and foliation stand in close connection with each — 
other. The inference seems reasonable that the intense compres- 
sion of the masses, as maintained by Mr, Mallet, has been attended 
with the generation of sufficient heat to allow of the observed 
chemical and mineralogical rearrangements. 
In some places the schistose structure disappears and is replaced 
by one of a thoroughly amorphous kind, indistinguishable from that 
of ordinary eruptive rocks. Where this has taken place veins or 
injected portions of the amorphous rock may be observed penetrating 
the adjoining highly crystalline foliated masses. It is in such cases 
difficult to avoid the conclusion that these intrusive veins are really 
portions of the foliated rocks reduced to the ultimate stage of 
erystalline rearrangement, every trace of foliation or original struc- 
ture having been effaced, and the rocks having been brought into 
a plastic condition, in which, during the crumpling of the crust, they 
were actually forced into cracks of the less highly altered members 
of their own series. There is no essential distinction between gneiss 
and granite, save the foliated structure of the one and the amorphous 
structure of the other. But gneiss in a plastic state and squeezed 
into fissures, or between beds of firmer consistence, would doubtless 
consolidate as granite. 
Thus the study of metamorphism and metamorphic rocks leads 
us from unaltered stratified deposits at the one end into true eruptive 
masses at the other. We are presented with a cycle of change 
wherein the same particles of mineral matter pass from igneous rocks 
into sedimentary deposits, then by increasing stages of alteration 
back into crystalline amorphous masses like the original rocks, 
whence, after being reduced to detritus and re-deposited in sedimen- 
tary formations, they may be once more launched on a similar series of 
transformations. ‘he phenomena of metamorphism appear to be 
' “Geologie Expérimentale,”’ p. 410. 

