7« PALiEONTOLOGY OF NEW-YORK. 



'^V^lateve^ may have been the cause of metamorphism or crystalline 

 etructure, 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 palaeozoic 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 may be, 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 Lkurcntian 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 wo do not yet know of any extensive forma- 

 tions of more ancient date. 



