Geographical Distribution. 23 



of the stone found lying on the surface or in the surface earth. They 

 are sometimes called " field stone." Having been transported many 

 miles and been subjected to great wear, they represent the more re- 

 sisting and harder rocks. They are generally more or less rounded 

 and scratched or furrowed. Geologically they are known as bould- 

 ers. On Long Island they are the only stone to be had. And some 

 of the older houses on the western end of the island are built of sand- 

 stone obtained from the drift. Others are of sandstone, trap-rock 

 and gneissic rock, mixed. In grading and cutting down the hills in 

 Brooklyn enough stone is sometimes found to lay the foundation 

 walls. Of course the drift includes the harder sandstones, quartzites 

 and gneisses mainly.* At Medina, in uncovering the sandstone 

 transported blocks occur in the stripping or drift. But this forma- 

 tion cannot be considered as a source of any regular quarry business, 

 though, in the further clearing up of the country, it may yield a great 

 deal of stone for the localities where it occurs. 



Slates. 



Slate is used to designate compact, fine-grained rocks, which have 

 the property of cleaving into thin plates. The term rests for its 

 signification upon a physical property rather than upon chemical or 

 mineralogical composition. Hence there are many varieties from the 

 argillaceous (clay-slate, or argillyte), to hornblendic, chloritic and 

 hydromica slates or schists. But nearly all of the slate which is em- 

 ployed either as constructive or as decorative material belongs to the 

 argillaceous variety, clay-slate. It is a sedimentary rock and occurs 

 associated generally with sandstones and other fragmental rocks. 

 Since the great use is for roofing material, it is often called roofing 

 slate. The prevailing colors are gray to blue-black ; purple, red, 

 green and variegated (red and green), are less common shades. There 

 is much variation in the degree of hardness, and some are very hard 

 and also brittle. The fineness of grain, or texture is another mark 

 of difference in slates. Generally the rock which is more highly 

 metamorphosed, is stronger and more fissile, and hence a more dura- 

 ble material. The direction of the cleavage may coincide with that 

 of the dip of the bedding planes, or it may be oblique to them. 

 For use as roofing material, the color, fineness of grain, strength, 

 hardness, freedom from pyrite or seams of calcite and quartz, and 

 durability are essential qualities. 



* At Yonkers the trap-rock boulders have been used largely in the building of re- 

 taining walls and foundations. 



