278 Pirsson — Phenocrysts of Intrusive Igneous Rocks. 



rock constituents, and when this dike is one of a set filled by 

 the same injection which may or may not have similar pheno- 

 crysts, we are thoroughly justified in assuming that its pheno- 

 crysts are formed in place and are not intratelluric. 



Evidences of intratelluric origin. — We should now consider 

 the evidences on which is based the view that phenocrysts 

 have been formed at great depths under wholly different con- 

 ditions from the other rock constituents. So far as the writer 

 can discover, they are these, — the contrast in size and crystal 

 form of phenocryst to the constituents of the ground mass, the 

 fluidal arrangement the phenocrysts sometimes possess and the 

 fact that they are often corroded or changed in character, that 

 is resolved. 



But the contrast of form and size, as we shall presently 

 show, may be explained in quite a different way ; the fluidal 

 arrangement certainly shows that the phenocrysts were 

 formed before the magma came to rest, but it does not show 

 how long before or that it may not have been just previously ; 

 while the corroded and altered crystals show a change of con- 

 ditions, not necessarily a change of place, and although this 

 may be a logical inference it is still not the only one. 



From the facts at our command, therefore, it appears most 

 reasonable to say that in some cases we know that phenocrysts 

 have been formed in place; in other cases we know that they 

 were formed prior to injection, but how long and how far we 

 do not know, and that in the great majority of cases we do not 

 know whether they were formed in place or not. It is the 

 author's belief that the majority are formed in place for reasons 

 which it is now proper to give. 



Conditions governing the formation of rock structure. — So 

 far as the writer can understand, the French believe in two 

 distinct and different periods of consolidation for all intrusive 

 rocks both granular and porphyritic ; many if not most of the 

 Germans believe in one for the granular and two for the por- 

 phyritic ; like some petrographers, the writer sees no necessity 

 for more than one in both. 



The conditions governing the consolidation and crystalliza- 

 tion of an igneous magma are complex, and many of them, 

 perhaps all, are as yet ill understood. The fall of temperature 

 is the most important, but the chemical composition of the 

 magma, the influence of mineralizing vapors and the pressure 

 are also of great importance. Another important considera- 

 tion which has been greatly overlooked is increasing viscosity, 

 which depends on chemical composition and temperature. It 

 would be impossible to discuss all these in full, and the writer 

 therefore confines himself to a brief statement of his own views. 



The greatest determinant in the formation of rock structure 

 is the ratio of time in the fall of temperature between the 



