QUARRY MATERIALS OF NEW YORK 1 23 



The difference of color is of importance economically. ' The red 

 forms decay rapidly, while the blue, though often becoming stained 

 yellow by iron, do not appear to disintegrate. The writer has not 

 been able to follow up this investigation as far as he could have 

 wished, and the discussion in this paper should be regarded as 

 merely preliminary to a more detailed presentation of the subject. 



The inference in the above quotation that the color variation has 

 significance with respect to the durability or weathering qualities of 

 the granite claims attention, though no explanation is vouchsafed in 

 the paper. The present study has not afforded any clear evidence 

 of such relationship. There is apparently a wide difference in the 

 capacity of the granite to withstand disintegration, but this feature 

 seems more related to the textural characters than to any peculiar- 

 ities of the mineral constituents that are reflected in the color. 



Some natural surfaces are practically fresh, though they have 

 been exposed to atmospheric conditions since Glacial time. In other 

 places the granite is disintegrated to some depth. The first stages of 

 weathering are usually manifested in a weakened cohesion of the 

 mineral particles, as the result of the alternate expansion and con- 

 traction under varying temperatures. The microscopic cracks and 

 pore spaces are enlarged with the progress of weathering. The 

 final stage of this physical disintegration is reached when the rock 

 becomes a loose, mealy aggregate of quartz, feldspar and mica. 

 Chemical decay, of course, accompanies the physical breakdown 

 and is first evidenced in the separation of iron oxide and the soften- 

 ing of the feldspar, but it is mainly effective after the rock has 

 undergone partial disintegration. 



It is evident from a study of the granite in the field that the 

 texture has much to do with its weathering qualities. The types 

 which are characterized by a closely knit fabric, with the individual 

 grains well interlocked, as observed in most unchanged granites, 

 are resistant to weathering. Such textures are found in the massive 

 varieties of the rock and in the foliated types which have not under- 

 gone noticeable granulation from shearing action. The granular 

 even-textured types, on the other hand, are apt to be of more 

 porous nature and more prone to disintegrate. 



The Yonkers is quite free of knots and streaks arising from ir- 

 regular mineral distribution. The principal variation relates to 

 texture and grain. Coarse, massive phases occur here and there as 

 a kind of pegmatitic development. Some exposures are only 

 moderately foliated. The characteristic rock, however, is thinly 

 foliate, with the biotite interleaving the quartz and feldspar at 

 rceular intervals. 



