Building Materials, Their Use and Origin 407 



193. Sandstone. Sandstone is easily quarried and cut 

 into shape for building purposes. The irregular blocks which 

 come from the quarries are dressed either by hand or by ma- 

 chinery. The stone m'ason is then in about the position of the 

 carpenter who has had most of his sawing and planing done at 

 the mill. Sandrock has various colors depending upon the 

 presence of minute quantities of iron and other mineral matter 

 in it. It is often used in thin slabs to form facings of rough 

 walls, or carved into various shapes for interior decorations. 

 A certain kind of sandstone found in Ohio hardens on exposure 

 to the air. This property gives it a wide market. 



Formation of sandrock. Notice the resemblance of a piece 

 of sandstone to sand. It was sand before it was made rock. 

 Sand with traces of lime was deposited as sediment in water in 

 a region which was gradually sinking. Rocks formed in this 

 manner are said to be sedimentary. Other deposits of clay, 

 sand, mud, or volcanic lava buried the sand layer or stratum 

 (strata, plural) and with their increasing depth subjected the 

 sand to ever greater pressure. Under a blanket which may 

 have been thousands of feet thick, the sand stratum was 

 changed in the course of untold ages by the pressure, and 

 was welded and cemented into sandrock. Then it was lifted 

 above the waters by titanic forces involved in the shaping 

 of the earth's surface and was left in the positions we find 

 it to-day. 



194. Slate. Familiar uses of slate are for roofing and for 

 blackboards. For both of these uses it is unsurpassed. The 

 peculiar advantage of this rock is that it car; be split along a 

 certain plane about as easily and as smoothly as a piece of wood. 

 This is the result of the heavy pressure to which it has been 

 subjected in the course of its formation, and not to the fact 

 that the original clay was deposited by the water in layers or 

 strata. 



Slate quarries may be from 200 to 300 feet deep. The blocks 

 after being taken out are sawed into the desired dimensions 



