/nON OF RCX 



>f these minerals, and subject 



them to careful examination, so as to fix their characters 

 mmd ; while at the s.n ;ll not omit tu 



devote as much time as he can spare to the atu 



of any good ntincralogical cabinet within hi* reach. 

 minerals which form essential constituents 

 ed rocks is comparatively small Nor arc 

 those very numerous which occur abundantly as im- 

 portant accessory or accidental ingre*: >apter 

 \\ 1 the reader will find a list of those which it is 

 desirable that he should know, with a reference to the 

 pan they play as constituents of rocks. 



Hut it' the geologist means to devote himself to the 



the genesis of rocks, particularly those of 



igneous and metamorphic origin, he will find it needful to 



h more fully into the domain of MINERALOGY. 



A ill he regret such an excursion; : 'lying 



the structure and growth of minerals he learns how rocks 



have been formed, and by what processes they have been 



altered since their formation. This is well brought out 



by the microscopical examination of crystals, as will be 



pointed out r 



Though practice alone can give the learner justifiable 

 ieiu e among rocks in the field, some hints may be 

 offered here for his guidance. He must learn to 



between essential and I :ers. Two 



rocks for : may exactly resemble each other in 



colour, and even in shade of colour, yet the one may be 

 . e or sedimentary mass, the other an original or 

 igneous one. Colour, therefore, can hardly be a very trust- 

 worthy index of the true nature of a rock (but see p. 68). 



