124 THE BOOK OF NATURE STUDY 



not the exterior shape of crystals. Thus a granite is made of 

 perfect crystals of the minerals felspar and mica, bound together 

 by crystalline quartz. The texture in this case is generally coarse, 

 and the individual minerals can be readily recognised the felspar 

 by its white or pink colour and its apparently irregular cleavage 

 planes, the mica by its brilliant pearly-looking cleavages and its 

 occasional hexagonal shape, and the quartz by its glassy aspect 

 and absence of either crystalline shape or cleavage. 



Basalt, like that of the Giant's Causeway, is a darker, heavier, 

 and finer-grained rock, which requires microscopic examination. 

 It is then found also to consist largely of felspar, with the addition 

 of olivine, iron-ores, and augite, the last being usually the least 

 perfect in shape, and the last substance to crystallise. The presence 

 in these rocks of perfect crystals suggests that they should be 

 called crystalline rocks, and the crystalline minerals present are 

 usually silica (quartz), silicates, or oxides. Such minerals are 

 insoluble in water and not easily decomposed by it, but they are 

 fusible by heat, and most of them have been experimentally 

 crystallised from a state of fusion. 



It is therefore practically certain that this class of rocks has not 

 been produced by water but by heat, and that they have been 

 injected in a fused state into their present position, cooling and 

 crystallising where they are now found. This supposition is sup- 

 ported by the fact that frequently sediments and other rocks 

 with which they come into contact are altered, and sometimes 

 recrystallised, by reason of the great heat brought to bear on them. 

 It is well known that the size and perfection of crystals is related to 

 the time they have taken to grow, and the uniform conditions under 

 which they have been maintained during growth. Different crystal- 

 line rocks exhibit marked differences in the size of their constituent 

 crystals, and it is naturally inferred that those with the largest and 

 most perfect crystals have solidified most slowly and at a consider- 

 able depth under the earth's surface, while those with less perfect 

 and smaller crystals have solidified nearer to the surface. 



Melted material having the composition of one or other of the 

 crystalline rocks is known to reach the surface in volcanoes, where 

 it is poured out in the form of lava, or shot out by steam in the 

 form of dust or ashes. When the lava cools it develops crystals, 



