464 



Scientific Geology. 



to sienite in their characters, and recede farther from those of slate, 

 the greater the quantity of feldspar. What can this circumstance re- 

 sult from, but from the greater degree of heat to which such nodules 

 have been subject 1 Their losing in so great a degree the slaty charac- 

 ter proves that they have been more nearly melted. 



Another circumstance lends in my opinion a plausibility to the pre- 

 ceding suggestions. Towards the northern extremity of the horn- 

 blende slate above described, and at least a mile distant from the sien- 

 ite, we find the slate composed of compact feldspar and hornblende ; 

 and its schistose structure almost obliterated. There is also a tend- 

 ency in the rock to divide into columnar and rhomboidal forms. Now 

 in these facts we see, it seems to me, the effects of a heat sufficient to 

 produce a partial fusion of the rock, but not an entire obliteration of 

 the slaty structure : sufficient for the production of feldspar, but not 

 for its crystalization. All this indicates a source of heat of great pow- 

 er at a small depth where probably genuine sienite exists. 



There is another fact which I have noticed in one portion of this 

 sienite, that lends still farther support to these views. Two miles 

 south of the spot where the sienite and slate meet, I observed the 

 traces of an obsolete stratification in the former rock, running in the 

 same direction as the basset edges of the slate. There is no actual 

 division of the sienite into parallel portions, but only the marks of a 

 former division by a sort of segregated ridges. The existence of 

 the nodules of slate in the sienite proves that the fusion of the rock 

 was never complete ; and in these faint traces of original stratification 

 do we not see evidence of the same fact ; and in the coincidence of 

 direction between the strata of hornblende slate and these marks, 

 have we not presumptive evidence of the origin that I have imputed 

 to the sienite. 



These various facts and inferences have led my own mind to make 

 another inquiry 1 Do we not here see the reason why one part of a 

 deposit is sienite and another part granite ; that is, a rock destitute of 

 hornblende ? When the fusion of a rock is complete and the heat car- 

 ried to a certain degree, may not the production of hornblende be- 

 come impossible, because those affinities and polarites operate that 

 produce other minerals, especially feldspar and mica ? The sienite 

 in the valley of the Connecticut at least, occupies a position generally 

 between the granite and the newer stratified rocks. And if we sup- 

 pose the heat to have been greater at the time of the production of 

 these rocks in proportion to the depth beneath the surface, it is obvi 



