MINERAL RESOURCES OF THE STATE OF NEW YORK 243 



shales and slates, that outcrop in long north-south belts. The 

 workable slates are mostly found in the upper part of the group and 

 near the overlying Ordovician strata, although folding may cause 

 them to appear in the middle part of the belts. These Cambrian 

 slates are mainly developed on the Vermont side and yield such 

 varieties as sea green, unfading green, purple and some variegated 

 slates. They are to be seen also in a series of quarries that extends 

 northward from North Granville, N. Y. 



The Ordovician strata include grits and red, green and black 

 slates and shales. They lie in parallel belts with the older series, 

 with which they are interfolded. Red slates are the characteristic 

 product of this group, and they are rather irregular in their distri- 

 bution and of uncertain continuity, running into shales or green 

 slate of inferior quality. 



The field relations of the slates are complicated by the intricate 

 folding and by deep erosion which has removed much of the over- 

 lying Ordovician strata. According to Dale they were subjected to 

 two periods of compression and folding, the first coming at the close 

 of the Ordovician and effecting the cleavage foliation and develop- 

 ment of grain in the clayey sediments, as well as new mineral com- 

 binations, particularly muscovite and chlorite. In the coarser sedi- 

 ments, like sandstone, quartz was deposited as cement and in veins. 

 In the later period of compression slip cleavage was induced on the 

 slaty cleavage, and minor faults, shear zones and diagonal joints 

 were produced. New infiltrations of silica and carbonates took place 

 in the openings made at this time, and along some of the fractures 

 came intrusions of igneous material which consolidated as dikes. 

 Erosion by atmospheric agencies or by ice has been operative since 

 the late Paleozoic and has carved the present surface out of the 

 folded strata. 



The cleavage is a variable quality that conditions largely the 

 success or failure of a quarry. Abrupt changes in the cleavability 

 of the material are not infrequent and the cause of much waste 

 in quarrying. As a rule the planes of cleavage are inclined at fairly 

 high angles, usually in excess of 45°. They are seldom or never 

 coincident with the bedding where this can be distinguished, which 

 it often can be by the variations of texture or color or by fossil 

 imprints. 



Varieties of slate. The best known product of the New York 

 quarries is the red slate that is practically restricted to Washington 

 county. Its color, a hematite red, and its somewhat silky lustre are 

 characteristic. It finds a ready market for roofing material at 

 fancy prices, for the supply is limited and the quarrying of it an 



