4l2± descriptions of rocks. 



almost all kinds of igneous and metaniorphic rocks, includ- 

 ing trap or doleryte. 



For a particular account of the distinguishing character- 

 istics of minerals studied by microscopic methods, reference 

 must be made to treatises on the subject. 



IV. KINDS OF ROCKS. 



1. Rocks are generally mixtures of two, three, or four 

 prominent mineral constituents, with also others, it may be, 

 of less importance. Each mineral adds a distinctive fea- 

 ture, and might be a reason for a new name. But it is 

 usual with lithologists to base the distinction into kinds of 

 rocks on the two chief minerals, and make the others acces- 

 sory species and the basis only of varieties. This method 

 is prompted by convenience, and also by the fact that the 

 more important characteristics are commonly contained in 

 two of the constituent minerals. It has many exceptions, 

 however, and particularly where a third mineral has special 

 peculiarities and abundance. 



2. Difference in kind of rock is naturally based on dif- 

 ference in chemical or mineral constitution, and identity, 

 accordingly, on essential identity in this respect. Conse- 

 quently when there is no essential difference in chemical or 

 mineral constitution, there is no sufficient reason for a dis- 

 tinction in kind or a difference in name, unless the wide 

 distribution of a particular variety, and the. permanence in 

 its characters, make the distinction in name a geological 

 necessity. 



In accordance with this statement, the distinctions among 

 crystalline rocks of coarse or fine in texture ; of being por- 

 phyritic or not ; of containing glassy grains among the 

 stony or not ; of being foliated or not 'in crystallization, are 

 of little value compared with the real mineral constitution, 

 and are a fit basis only, at the best, for varieties. But the two 

 rocks of like composition, trachyte and felsyte, retain their 

 characteristics so widely, that geolosry needs both names, 

 and only demands that their essential identitv should be 

 held in mind. 



The same kind of rock is in many cases both of metanior- 

 phic and eruptive origin ; still the difference of origin is 

 not a sufficient basis for a distinction of kind unless there 

 is some marked difference between them, and an extended 



