﻿l8o NEW YORK STATE MUSEUM 



The amount of ferric oxid in the red feldspar is undoubtedly 

 very trifling, so that, if chemical combination has taken place and 

 the lime has entered into the reaction the quantity involved is so 

 small that it would be a comparatively insignificant feature in the 

 complete rock analysis. It is to be noted that the lime is some- 

 what higher in analysis i than in 4, but, while it is possible that 

 this is owing to lime taken up from the limestone and going into 

 combination with the iron of the feldspar, it must be remembered 

 that the variation is well within the limits of variation which all 

 the bases show in the general granite mass, hence it is absolutely 

 unsafe to generalize in regard to it. We may have a combination 

 of lime, iron and alumina in a spinellike mineral, though lime does 

 not, in general, occur in spinel; or a small amount of anorthite 

 may be formed, with the iron replacing alumina. The iron may 

 perhaps be reduced, forming an iron aluminate, the iron reduced 

 to the ferrous condition, though it would seem as if this would 

 likely give a green color to the feldspar. Warth argues that his 

 color changes need not mean chemical combination of the two oxids 

 but rather a diffusion of one in the other. * Hillebrand, however, is 

 quite confident that combination takes place. He says, " It is un- 

 questionable that both lime and alumina decolorize and combine 

 with ferric oxid when they are heated together." Though this 

 chemical question must be left indefinite, it does seem to us cer- 

 tain that the red color of the feldspar may be made to disappear 

 merely by sufficiently high and prolonged heating, that the presence 

 of lime facilitates the process, lowering the necessary temperature, 

 and that with our feldspars here the temperatures were not suf- 

 ficiently high to discharge the color, or rather to cause the feldspars 

 to crystallize with the iron combined, rather than as free hematite, 

 under the conditions prevailing at the place latnd time Of solidification ; 

 though they were high enough to cause the combination to take 

 place when in the vicinity of limestone and under its influence. 



The only difference in the mineralogy of the two rocks is the 

 presence in the white rock of occasional, scattered, small black 

 tourmalins, which in general do not appear in the other, though 

 they are locally present even there. They Would seem attributable 

 to the presence of mineralizers in the border phase of the granite, 

 and in the dikes they occur in the .red granites adjacent to other 

 rocks than limestone, such as quartzite for example, and seem to 

 have nothing whatever to do with the color change. It seems to 

 us that the chemical analysis gives no suggestion whatever as to 

 the cause of this change. 



