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Sect. VI. 1 



GEOLOGY. 



159 



aspect of the feldspar and quartz in granite, lie ^^ ill know 

 tlie two most essential ingredients in most igneous rocks ; 

 and in granite he will often find the glittering scales of 

 mica replaced by a dark green mineral, less hard than 

 the feldspar and quartz; and then he 



will Know me 



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thlrd most important mineral, hornblende. 



mentary rocks can hardly be described, except by the 



terms in common use : impure limestone, which cannot 



be readily recognised by the eye, can be distinguished 

 by its effervescence with acids. By the repeated compa- 

 rfson of freshly fractured sedimentary and igneous rocks, 

 such as sandstone and clay-slate on the one hand, and 

 granite and lava on the other, he will learn the difference 

 between crystalline and mechanical structure ; and this 

 is a very necessary point. Let no one be deterred from 

 geology by the want of mineralogical knowledge ; 

 inany excellent geologists have known but little ; and 

 from this reason its value has perhaps sometimes been 

 underrated, for many of the obscurer points in geology, 

 such as the nature of the metamorphic changes in rocks, 

 and all the phenomena of metallic and other veins, 

 almost require such knowledge. The appearances pre- 

 sent. J by the different forms of stratification (that is, the 

 original planes of deposition) may be soon learnt in the 

 field ; though no doubt the beginner would be aided by 

 the diagrams given in many elementary works. 



The two most useful works which the geologist can 

 carry with him, are without doubt the ' Principles ' and 

 the ' Elements of Geology,' by Sir Charles Lyell. He 

 should procure a treatise on mineralogy, for instance, 

 ' Philhps's Mineralogy,' by Allan. If he has the oppor- 



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