WATER-BEARING FORMATIONS 37 



Sandstone is a consolidated sand. It is said to be massive if 

 there are few bedding planes, and shaly if it splits into plates. 



Quartzite is a sandstone in which the spaces between the 

 grains have been filled with a hard cement (silica), forming an ex- 

 cessively hard rock. 



Shale is consolidated clay; a soft, fine-grained rock which 

 tends to split into thin plates. It is sometimes improperly called 

 soaps tone. 



Limestone is composed mainly of carbonate of lime, but often 

 contains sand and other impurities, and may be very hard. Not 

 infrequently it contains many shells or is made up entirely of 

 them. It can be most readily recognized by the bubbling which 

 takes place when it is touched with hydrochloric (muriatic) acid. 

 In varieties high in magnesia and in magnesium carbonate 

 (dolomite), hot, strong muriatic acid is necessary to produce this 

 action. 



Tuff is any sedimentary rock that is made up entirely or 

 almost entirely of fresh fragments of volcanic rocks. 



Concretions are hard, lumplike masses within the rock. 

 They should not be confounded with real boulders, from which, 

 as a rule, they may readily be distinguished, because they consist 

 of nearly the same material as that in which they are embedded. 



Igneous Rocks. Granite is a wholly crystalline rock, com- 

 posed of quartz, feldspar and other light-colored minerals. 



Diorite, gabbro and diabase are crystalline rocks similar to 

 granite, but with less quartz and with dark-colored feldspars. 



Volcanic rocks, lavas, etc., are rocks that have been emitted 

 in a molten state from volcanoes. 



Trap is a widely distributed, compact, very dark-colored 

 variety of volcanic rock. Technically it is a basalt or diabase, 

 and these terms are in common use. 



Metamorphic and Crystalline Rocks. Slate is like shale, but 

 harder; it splits into thin plates which may or may not coincide 

 with the bedding. The tendency to split is not often recognized 

 in drilling. Roofing slate is a familiar example. 



