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Re 
E 
336 Prof. Beck on Igneous Action, as exhibited in New York. 
On the contrary, when found in the mica slate this mineral 
almost invariably exhibits a perfect form and a fine finish. Such 
are the crystals from Dover, Dutchess County, and I might add 
those in the mica slate in Monroe, (Conn.) Delaware County, 
(Penn.) &c. 
From the facts which have now been presented, the conclusion 
seems to me almost irresistible, that whatever may have been the 
agency by which these minerals were originally segregated, the 
rocks in which they are found were subsequently subjected toa 
high temperature ;—sufliciently high at least, to soften many of 
the minerals imbedded in them. Thus we can account for the 
bent and rounded crystals of feldspar, apatite, quartz, scapolite, 
&e., so abundant i in many parts of the state, and for the similar 
nces presented by the garnet in gneiss. The mica slate 
having been farther removed from the supposed source of heat, 
has its imbedded crystals more perfectly developed. 
_ In many of these cases, the crystals were undoubtedly formed 
at first in obedience to the laws of crystallization. But we have 
no reason to believe that these laws were exerted so as to give 
rise to those irregularities of surface and structure, those contor- 
tions and fractures and glazings which they now exhibit. On 
the contrary, these appearances are entirely similar to those which 
we know to be produced, by subjecting perfect crystals enclosed 
in a sufficient quantity of sand or rock, to a high degree of heat. 
It has been thought by some geologists to be a necessary col- 
dition, that during the time these changes were effected the 
limestone must have been covered with water to have prevented 
the rock from undergoing calcination. It is well known, how- 
ever, that even in an ordinary kiln, it requires a very high heat to. 
calcine small masses of limestone, and unless some moisture is 
present, and layers of combustible matter interposed between 
those of the limestone, the evolution of carbonic acid is exceed* 
ingly sluggish. 
The pillars of the old Exchange in the city of New York, con- 
structed of white dolomitic marble, suffered little alteration by — 
the intense heat of the great fire of 1835, which raged for twenty- 
four hours, and destroyed more than six hundred houses. ‘They 
were somewhat disintegrated on the outside, and perhaps through- 
out became a little more granular, but there was no appreciable loss 
of carbonic acid. It seems to me, therefore, not unreasonable t0 
