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 to a 

 high temperature ; — sufficiently 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, 

 &c, so abundant in many parts of the state, and for the similar 

 appearances 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 con- 

 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 to 



