76 
PALAEONTOLOGY OF NEW-YORK. 
Whatever may have been the cause of metamorphism or crystalline 
structure, even in these old Laurentian rocks, it becomes clear that 
they were derived from pre-existing sedimentary strata ; and that 
masses of that prior formation, with its lamination still shown, are 
preserved in the most ancient crystalline rocks yet known upon the 
globe*. These facts furnish proof, moreover, that the mass has never 
been subjected to that high degree of heat which is usually supposed to 
accompany the production of crystalline granite. 
Returning, therefore, to the consideration of the metamorphic rocks 
of the Appalachian chain, we shall nowhere find evidence of extensive 
metamorphism produced by contact or proximity of a metamorphosing 
agent. The Laurentian rocks, on which rests the Potsdam sandstone, 
the lowest of the Appalachian series, had been metamorphosed, and the 
present mountain ranges formed long anterior to the time of the deposi¬ 
tion of this oldest of the paleozoic rocks. Even could we for a moment 
suppose it to be true that the contact of the granite or other so-called 
plutonic rock could change the entire mass through thousands of feet 
vertically, and many miles in extent laterally, beyond the limits of 
its contact, then we should necessarily expect to find that in the imme¬ 
diate vicinity of intrusive granites the surrounding mass would be more 
changed than at a distance from that point; but such is rarely the fact. 
Indeed at the present time few geologists, I think, are willing to main¬ 
tain that metamorphism has been caused, in any important degree or 
extent, by intrusive rocks. The influence, whatever it maybe, has per¬ 
meated the entire mass of sediments, independent of all surrounding 
influences, or of contact or proximity of heated or melted masses. 
That these mountain masses, in their great depression beneath the 
present sea level, may have reached a point where the surrounding 
temperature was much higher, is doubtless true, and this increase of 
* These rocks of the Laurentian mountains of Canada and the Adirondacks of New-York, from 
all their relations to newer formations, and in mineral association, are probably identical with the 
gneissoid and granitic rocks of the north of Europe, and we do not yet know of any extensive forma¬ 
tions of more ancient date. 
