10 THE MTNEEALS CONSTITUTING ROCKS 



But however great the adaptability of these few minerals may- 

 be, they are, nevertheless, subject to very definite laws of chem- 

 ical equivalence. There are elements which they cannot take 

 into their composition, and there are circumstances which retard 

 their formation while other minerals may be crystallizing. In 

 a mass of more or less accidental composition it may, there- 

 fore, be expected that other minerals will form in consider- 

 able numbers, but minute quantities. It is customary 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 the presence or 

 absence of which does not fundamentally affect its character, are 

 called accessory constituents. The accessory mineral which pre- 

 dominates, and which is present in such quantities as to be 

 recognizable by the unaided eye, is the characterizing accessory. 

 Thus a biotite granite is a granite 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 consoli- 

 dation of the mass or of subsequent changes. We thus have : 



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

 upon its first consolidation. All the essential constituents are 

 original, but, on the other hand, all the original constituents 

 are not essential. In granite, quartz and orthoclase are both 

 original and essential, while beryl and zircon or apatite, though 

 original, are not essential. 



(2) The secondary constituents are those which result from 

 changes in a rock subsequent to its first consolidation, changes 

 which are due in great part to the chemical action of percolat- 

 ing water. Such are the calcite, chalcedony, quartz, and zeo- 

 lite deposits which form in the druses and amygdaloidal cavities 

 of traps and other rocks. 



Below is given a list of the more important rock-forming 

 minerals, arranged as above indicated. Although these are 

 sufficiently described as regards their chemical and crystallo- 

 graphic properties in any of the mineralogies, it has seemed 

 advisable to devote some space here to a reconsideration of 

 those most prominent as rock constituents, in order that the 



