262 THE CANADIAN NATURALIST. [Vol. vi. 



Tourmaline, however, contains, besides some of the subtances just 

 mi^ntioned, boraoio acid and fluorine, and, in its mode of occurrence, 

 resembles such accessorial or accidental minerals as zircon, apatite^ 

 titanite and others. Gurnet, corundum, epidote, cordierite and. 

 scapolite are rock minerals, containing no other chemical consti- 

 tuents than those above mentioned, but they have been excluded 

 from our list because they resemble the accessorial constituents 

 m the manner of their occurrence. 



With reij-ard to these essential minerals it is first to be remarked 

 that the analyses which have been made of them are not, in 

 every case, of such specimens as have actually formed pirt and 

 portion of some rock species. To obtain pure specimens of the 

 minerals of rocks is often a matter of {i'reat difficulty, and well- 

 developed crystals from veins or geodcs have been preferred fox* 

 analysis to the generally amorphous particles of the same species 

 which enter into the constitution of rocks. The composition of 

 these minerals cannot, like that of well-crystallised artificial che* 

 mic.d compounds, be unequivocally expressed by chemical formulae. 

 Attempts, the most painstaking and persevering, have been made 

 in this direction by mineralogists, and the result has only been to 

 shew that, in the majority of cases, each analysis of the same 

 species demands a difijrent formula for expressing its composition 

 in chemical equivalents. The composition of micas, augites and 

 hornblendes is especially variable, and even with regard to the 

 felspars it has been maintained that those of our list are not dis- 

 tinct or independent species but are mixtures of one with the other 

 or with other supposed species.sueh as krablite,albite oradularia. 

 It has thererore been considered best here to neglect their various 

 assumed chemical formula and to regard principally their average 

 chemical composition. 



Certain difi"erences in the composition of these minerals cause 

 their subdivision into two different classes. The minerals of the 

 first class are mostly silicates of alumina, lime, potash and soda, 

 and it maij he called thefelsjmthic class. It includes, however, 

 leucito and ncphcline, which can scarcely be called felspars, and 

 quartz, which, although of very different composition, nevertheless 

 possesses lithological affinities connecting it closely with the acid 

 felspars. The minerals of the second class also contain lime, but 

 alumina and the alkalies are less frequent or absent altogether, 

 being replaced by magnesia and protoxide of iron. They are 

 generally of a more basic nature than the felspathic class, and the 



