CHAPTER XIII 

 IGNEOUS ROCKS 



THERE are many rocks in the earth-crust which do not bear 

 the characters described in the last chapter. They are of 

 different composition, texture, and structure, differently placed in 

 the crust, and they evidently originated in some other way than 

 by deposition. Granite and basalt are examples. They never 

 yield fossils, they are usually unstratified, and their composition 

 shows that they are not made up of broken fragments of organisms, 

 or of sand, mud, or pebbles, broken from pre-existing rocks. 



If rocks of this nature are accessible, their characters and rela- 

 tions should be studied. The absence of stratification and of fossils 

 in them tends to show that it is not likely that their constituents 

 have been dropped down on sea beds. Instead they break along 

 massive rectangular joints (Fig. 30) or into columns or spheroids, 

 structures comparable to those seen in starch or plaster or in some 

 other substance which has shrunk on cooling or drying. If their 

 relations to other rocks can be seen, these will show that they have 

 the shape of somewhat irregular masses encroaching upon other 

 rocks as if they had been injected into them when liquid and under 

 pressure, and that they must have consolidated from a liquid 

 state in the position where they are now found (Fig. 31). As the 

 masses expand in breadth downwards, and frequently end off 

 above by abutting against sedimentary rocks, they would seem 

 to have come up from below in a liquid condition. 



Examination of hand specimens of a rock of this kind, if 

 coarse grained by the naked eye or a lens, if fine grained in a 

 thin slice by means of a microscope, will show that, instead of 

 being made up of broken and rounded grains of sand or pebbles 

 or bits of organisms, they consist of crystals, some of them 

 quite perfect and complete in shape, fitting into one another or 

 into other substances which have the internal structure though 



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