OUARRY MATERIALS OF NEW YORK 



117 



the granite when examined microscopically appears little more 

 weathered than the gray variety. The apparently even distribution 

 of the coloring matter when the rock is viewed in the mass disap- 

 pears on closer examination and the stain is seen to be developed 

 in flecks and lines scattered over a white background of feldspar 

 and quartz. Most of the limonite is found in the quartz which is 

 the ingredient that shows the most granulation and consecjuently the 



Fig. II The yellow Mohegan granite, showing concentration of limonite 

 along the borders and in the cleavage cracks of the mineral particles 



most open space for its deposition. The source of the limonite is 

 traceable to iron-bearing solutions from the surface which found their 

 way downward along the joints and then diiTused through the rock 

 by means of the capillary openings. It may have been derived from 

 decay of the overlying rock in the long j^eriod of exposure previous 

 to Preglacial time, but of such a zone of disintegration there is no 

 remaining evidence at present and is hardly to be expected after the 

 erosive work of the ice. The limonite often seems to be concen- 

 trated about the biotite, but this is not a result primarily of a 

 chemical alteration of that mineral, but rather arises from the in- 

 filtration of the iron along the cleavage planes of the biotite. Much 

 of the biotite is perfectly fresh, showing no bleaching or other 

 change that could result in freeing any of the iron. In some of the 



