﻿46 NEW YORK STATE MUSEUM 



categories, effects produced upon the igneous rocks themselves, 

 effects of the igneous rocks upon the sediments whereby rocks of 

 intermediate composition are produced, and effects produced upon 

 the sediments by the injection into them of fluids from the igneous 

 rocks, fluids rich in mineralizing agents, and of quite different com- 

 position from the general mass of the igneous rock. 



Bleaching of granite by limestone. In the early stages of the work 

 it was noted that, while granite dikes and knobs of all sizes were 

 of frequent occurrence, cutting the Grenville limestone wherever 

 exposed, in all cases the granite was white, nearly as white as 

 the limestone in fact. The granite of the bathyliths is, however, 

 uniformly of red color, as are also the dikes in rock other than 

 limestone. This led to search for limestone contacts along the mar- 

 gin of the Antwerp bathylith and of the smaller granite intrusions of 

 the Theresa sheet, when it was found that in every case the margin 

 of the granite, adjacent to the limestone, was turned white. It also 

 proved to be the case in subsequent work that whenever, in passing 

 over granite country, a whitening of the rock was observable, 

 directly beyond crystalline limestone was sure to be found. It also 

 was found that the general granite of the Antwerp bathylith had 

 had singularly little contact effect upon the limestone, pure, 

 unchanged limestone lying directly in contact with the granite in 

 most cases, and that the dikes also had had no contact effects, so that 

 the rather unusual condition was presented of granite-limestone 

 contacts in which the granite was the rock showing contact effects, 

 not the limestone. 



Study of the white granite, both chemically and in thin section, 

 affords no explanation of the change. The white granite is in 

 general somewhat more acid than the red, but that is believed to be 

 nothing more than an expression of the general fact that the dikes 

 which radiate out from the bathyliths are more acid than the main 

 mass, whether they be red or white (they are usually red in all rocks 

 except the limestone), and that the granite also is apt to become 

 more acid near the margins. A little tourmalin is sometimes devel- 

 oped in the granite where white, but it also developed elsewhere. 

 The change seems to consist merely in a decoloration of the feld- 

 spar, changing it from red to white ; that of course on the as- 

 sumption that the red color of the feldspar is original and not 

 a later coloring due to slight alteration. In that case, however, 

 it is difficult to understand why both feldspars, of the white granite 

 as well as of the red, should not have undergone the alteration ; this 

 seems in fact so highly improbable, that we seem justified in regard- 



