﻿120 NEW YORK STATE MUSEUM 



quarries whose field geologic features give promise of encouraging 

 results. The most characteristic variations are illustrated in the 

 accompanying photomicrographs, plates 22, 23. 



Texture. The rock is granular, the individual grains varying 

 from minute particles in the finer shale layers to three or foul 

 tenths of a millimeter in diameter in the. coarser sandstone [pi. 23, 

 lower figure]. The grains are seldom rounded. Jagged or frayed 

 or elongate forms are the rule [pi. 2^, upper figure]. There is no 

 marked porosity. When the rock was first deposited as a sediment 

 it probably had the usual large interstitial spaces of such rock type, 

 but in this case some subsequent modification — an incipient meta- 

 morphism — has largely obliterated the voids by the introduction or 

 development of mineral matter of secondary origin. 



In general it is quite apparent that the average grain was orig- 

 inally more rounded than its present representative. 



Mineralogy. The original minerals in order of abundance 

 were the feldspars, quartz, and probably hornblende, biotite, and in 

 much smaller amounts others of little apparent consequence in the 

 present discussion. 



All of these have been more or less affected by subsequent 

 changes. Quartz has suffered least of all, the chief modification 

 being a greater angularity of form and an occasional interlocking 

 tendency caused by secondary growth [pi. 22, lower figure]. 



Both orthoclase and plagioclase feldspars occur. The orthoclase 

 grains, which originally made up more than half of the bulk of the 

 coarser types of rock, have been in places profoundly altered [pi. 22, 

 upper figure]. In many cases the identification of this mineral de- 

 pends upon its association and the abundant remnants of character- 

 istic structure and its normal secondary products. In the least 

 affected grains satisfactory identification is not difficult. Even in 

 the most modified representatives there is some preservation of 

 structure indicating size of grain and proving the essentially gran- 

 ular character of the rock. The plagioclase, although not abundant, 

 is more readily detected than the orthoclase because it has been 

 much less affected by the secondary changes. 



All original f erromagnesian constituents are w;holly altered. There 

 were some such constituents in the rock, as is plainly shown by the 

 secondary products. Hornblende and biotite were probably both 

 present. 



The secondary products, derived from the original feldspars and 

 ferromagnesian constituents, include sericite, chlorite, calcite and 



