114 MESSRS. J. E. DAKYNS AND J. J. H. TEALL ON THE 



microcline. Quartz comes in with orthoclase. We may therefore 

 arrange the essential minerals in the following order, based on their 

 distribution in the different types of rock in which they are most 

 important : — 



Olivine. 



Pyroxene. 



Hornblende. 



Biotite. 



Orthoclase and quartz, 

 Microcline. 



It is instructive to note that when the minerals pyroxene, horn- 

 blende, biotite, plagioclase, microcline, and quartz make their first 

 appearance, they play the role of groundmass ; in other words, they 

 are allotriomorphic or ophitic with respect to the other constituents.^ 

 It is only when a mineral has, so to speak, established itself as 

 an important constituent that it begins to show traces of idio- 

 morphism.^ 



This leads us to consider another point of great interest : namely, 

 the order of formation of minerals in individual rocks. 



The determination of this order is based on a consideration (1) of 

 the inclusions, and (2) of the relative perfection of crystalline form in 

 the different constituents. If the minerals in any rock-specimen 

 resulted from the consolidation of a magma having the same com- 

 position as the specimen, and if the minerals separated out one after 

 the other, each having its own definite period, there would be no 

 difficulty in determining the order in unaltered rocks. These con- 

 ditions, however, and especially the latter, are rarely realized. Two 

 minerals may be both idiomorphic and allotriomorphic with respect 

 to each other in the same rock. 



The facts indicate that the only order we can establish is that in 

 which the different constituents commenced to form. "We may sup- 

 pose that the whole period covered by the process of consolidation of 

 the rock is divided into a number of shorter periods, during which 

 each of the individual constituents was separating out. These 

 shorter periods overlap, and it is this overlapping which tends to 

 obscure the operation of the natural law. The overlapping reaches 

 its maximum in the plutonic rocks. 



If now we consider the most basic rocks we find that iron ores 

 preceded olivine, olivine preceded pyroxene, pyroxene preceded 

 hornblende and biotite, and all the ferro-magnesian constituents 

 preceded plagioclase. Prom the other rocks in the series we may 

 infer that plagioclase preceded orthoclase, microcline, and quartz. 

 We obtain then the following as the order in which the principal 

 constituents commenced to form : — 



^ This is well seen as regards pyroxene in 4020 ; biotite and hornblende in 

 4022 ; plagioclase in 4023 ; microcline in 4034 ; quartz in 4027. See page 112, 

 suj)ra. 



^ This remark applies only to those minerals which enter largely into the 

 composition of the rocks. 



