CHAPTER XII 

 CLASTIC ROCKS 



THE examination of specimens of rocks collected from quarries, 

 mines, or cuttings, or from road metal or building stone, requires 

 in most cases the use of a magnifying glass, and sometimes 

 microscopic slides must be prepared. Other observations may be 

 made by breaking the rock to pieces, washing it in water, acting 

 on it with acid, and, where necessary, analysing it chemically. 

 It is found that the ultimate constituents of all rocks are minerals, 

 chemical compounds capable of crystallising, and sometimes found 

 in rocks in crystalline shapes. In certain rocks some of the minerals 

 occur as crystals with their own shapes, and such rocks may be 

 called crystalline ; others have their minerals, not as perfect 

 crystals but as broken fragments of what have once been crystals, 

 and so may be called fragmental or clastic. Granite and basalt 

 are examples of the first class, sandstone and conglomerate of 

 the second. 



It is best to begin with the consideration of the clastic rocks, 

 and no better type for the purpose exists than conglomerate 

 (Fig. 25). This is at once seen to be made of rounded pieces 

 of all sorts of other stones, granite, basalt, flint, limestone, sand- 

 stone. The pieces are of all shapes, but their edges and angles are 

 rounded. If broken out of the rock they are undistinguishable 

 from shingle or gravel pebbles ; they are made of the same kinds of 

 rocks and are of about the same shape and size (compare Figs. 25 

 and 17). They are set amongst finer pebbles and chips of similar 

 rocks, with sand or mud between, and all is bound up by some 

 kind of solid cement. The whole thing is very like concrete which 

 is made of pebbles and sand bound into a solid mass with an arti- 

 ficial cement prepared from lime. If the fragments of such a rock 

 are angular, like the fragments found in screes, the rock is called 

 a breccia. 



