6o 



OUTLINES Ol 1 I ! 





i. CRYSTALLINE. In this type the rocks have a granu- 

 lar structure, and on inspection t he apparent 

 found to be crystals, or crystalline particles, so inter- 

 mingled, or felted together, as to give coherence to the 

 stone. In the coarse-grained varieties, lik< many ur.. 

 the crystals of which can be distinctly seen at a di^ 

 of several yards, their true crystalline nature is at once 

 apparent We see that their grains are all crystalline, 

 and that the lustre reflected from so many shining points 

 on their surface comes from the cleavage planes of the 

 component minerals. But as the texture becomes finer, as 



it does, for example, 

 in the family of the 

 basalt-rocks, the un- 

 assisted eye may 

 hardly be able to de- 

 tect any crystalline 

 facettes, even on a 



Fir.. 7. A piece of pranite. Crystalline structure, fresh fracture. The 



lens, however, with 



often show that such rocks really consist of very small 

 crystals. But the fineness of grain may reach such a point 

 as to escape detection even by that means, and then the 

 observer must call the rock a compact one. It may still be 

 quite crystalline, however, when examined under the micro- 

 scope, in the manner described in a later chapter. We are 

 at present concerned only with those external characters 

 which can be recognised by the observer in the field. 



Viewed broadly the crystalline particles are found to 

 be built up on two different plans. In the great majority 

 of rocks they are (i) amorphously aggregated, that is, 



