IXX PEOCEEDINGS OF THE GEOLOGICAL SOClEXr. [May I9OI, 



directed to the mutual relations of the constituents of granite and 

 to the inferences which could be drawn from a study of those 

 relations as to the order of consolidation of the minerals. 



The Consolidatiok op Igneous Magmas. 



At the time when the discussion took place this kind of reasoning 

 could be applied only to coarse-grained rocks, but with the advent 

 of the microscope it became possible to extend it to the important 

 group which had been designated, in the earlier classifications, as 

 ' apparently homogeneous rocks.' In the early part of the century 

 Cordier had proved, by the microscopic examination of the powder 

 of basalt, that this rock was heterogeneous, but it was not till the 

 examination of thin sections had been introduced that tbe mutual 

 relations of the constituents of the finer-grained rocks could be 

 studied. In those rocks which have resulted from the consolidation 

 of homogeneous silicate-magmas, and in which the consolidation has 

 been unaccompanied by the phenomena of resorption — that is, in 

 which there has always been equilibrium between the constituents 

 during the process of consolidation — the order of separation can be 

 inferred from the microscopic structure. 



It has thus been established that the process of consolidation 

 cannot be divided into a number of sharply-defined periods, each 

 characterized by the separation of some one mineral only ; but that 

 the times during which the different minerals are separating out 

 overlap to some extent. The amount of overlapping varies in 

 difierent cases, and, in one and the same magma, is most marked in 

 plutonic masses ; whence we conclude that it is largely determined 

 by physical. conditions, and especially pressure. 



The order of consolidation, as determined by an examination of 

 the mutual relations of minerals, is, therefore, the order in which 

 they commence to form, and this order may or may not agree 

 with that in which they cease to form. The laws which express 

 the order of formation of minerals, and the chemical and physical 

 conditions which control that order, have not as yet been definitely 

 established. 



One of the most important papers on theoretical petrology is 

 undoubtedly that by Prof. Rosenbusch on the significance of the 

 granular and porphyritic structures in massive rocks. The import- 

 ance of this paper must not be judged simply by the amount of 

 truth in the principles enunciated, but rather by the stimulus 

 which it i?ave to theoretical considerations and to researches 



