HANDBOOK FOR THE DEPARTMENT OF GEOLOGY. 509 



erals of the hornblende or pyroxene group cau be formed in a mass 

 cooling from a state of fusion ; they may be crystallized from solution, 

 or be formed from volatilized products. They are therefore the com- 

 monest of minerals and rarely excluded from rocks of any class, since 

 there is no process of rock formation which determines their absence." 

 Moreover, most of the common minerals, like the feldspars, micas, horn- 

 blendes, pyroxenes, and the alkaline carbonates possess the capacity 

 of adapting themselves to a very considerable range of compositions. 

 In the feldspars, for example, the alkalies, lime, soda, or potash may 

 replace each other almost indefinitely, and it is now commonly assumed 

 that true species do not exist, but all are but isomorphous admixtures 

 passing into one another by all gradations, and the names albite, oliglo- 

 clase, anorthite, etc., are to be used only as indicating convenient stop- 

 ping and starting points in the series. Hornblende or pyroxene, further, 

 maybe pure silicates of lime and magnesia, or iron and manganese may 

 partially replace these substances. Lime carbonate may be pure, or 

 magnesia may replace the lime in any proportion. These illustrations 

 are sufficient to indicate the reason of the great simplicity of rock 

 masses as regards their chief constituents, and that whatever may be 

 the composition of a mass within nature's limits, and whatever may be 

 the conditions of its origin, the probabilities are that it will be formed 

 essentially of one or more of a half a dozen minerals in some of their 

 varieties. 



But however great the adaptability of these few minerals may be they 

 are, nevertheless, subject to very definite laws of chemical equivalence. 

 There are elements which they can not take into their composition, and 

 there are circumstances which retard their formation while other min- 

 erals may be crystallizing. In a mass of more or less accidental com- 

 position it may, therefore, be expected that other minerals will form, in 

 it may be, considerable numbers, but minute quantities.* It is custo- 

 mary to speak of those minerals which form the chief ingredients of 

 any rock, and which may be regarded as characteristic of any particular 

 variety, as the essential constituents, while those which occur in but 

 small quantities, and whose presence or absence does not fundamentally 

 affect its character, are called accessory constituents. The accessory 

 mineral which predominates, and which is, as a rule, present in such 

 quantities as to be recognizable by the unaided eye, is the characterizing 

 accessory. Thus a biotite granite is a stone composed of the essential 

 minerals quartz and potash feldspar, but in which the accessory mineral 

 biotite occurs in such quantities as to give a definite character to the 

 rock. The minerals of rocks may also be conveniently divided into two 

 groups, according as they are products of the first consolidation of the 

 mass or of subsequent changes. This is the system here adopted. 

 We thus have: 



(1) The original or primary constituents, those which formed upon 

 its first consolidation. All the essential constituents are original, but 



* Hawes, the Minerals of Building Stones, Rep. Tenth Census, vol. x, p. 4. 



