312 PROCEEDINGS OF THE AMERICAN ACADEMY. 



any case there can be little doubt but that the quartz and feldspar 

 were the first minerals to crystallize. Owing to the very slow cooling 

 (long time period) there was alnmdant time for a perfect equilibrium 

 to establish itself, no supersaturation ensued, and the minerals con- 

 tinued to grow chiefly by addition to crystals already formed and the 

 crystallization of all these proceeded, accompanied as we shall see by 

 certain transformations, chiefly in the feldspar, throughout the re- 

 mainder of the main (magmatie) period of crystallization. Succeed- 

 ing this, and continuous with it, certain changes were effected by the 

 action of late mineralizing vapors or solutions, later growths of albite, 

 aegirite, quartz and zircon being evidence of this. In fact no line 

 can be drawn marking the close of the main period of crystallization, 

 and there is doul)tless in all magmas a slight but continuous mineraliz- 

 ing action going on for a long time after the main crystallization has 

 virtually ceased, as the temperature remains relatively high for a long 

 period, and the water, etc. have not been wholly eliminated. 



Zircon appears to have grown throughout the entire period of crys- 

 tallization but to have especially favored the last stages. This agrees 

 with the observations of others on the zircon of similar rocks. ^^ 



The element of time appears here as the great conditioning factor. 

 A long period of time made the establishment of a relatively perfect 

 ec[uilibrium possible and so smoothed out, as it were, the cooling 

 curves of the magma, and left in the textural relations of the vari- 

 ous minerals little or no trace of critical points in the progress of the 

 crystallization. '^'^ 



In the fine-granite, found as the contact phases in the eastern and 

 southeastern parts of the area, we find in the texture good evidence 

 that these contacts were at lower levels of the original contact than 

 were those of the granite-porphyry. The rock is somewhat porphy- 

 ritic, the phenocrysts being microperthite and quartz. ThCiSe merge 



69 See Murgoci, op. oit., p. 137; also Warren & Palache, op. cit., p. 144. 



70 In this connection the writer wishes to direct attention to the discussion 

 of the devcIoi>mcnt of the porphyritic texture given by Professor Crosby in 

 his Bhie Hill report (op. cit., pp. 360-1), and in a later paper (On the Origin of 

 Phenocrysts and the Development of the Porphyritic Texture in Igneous 

 Rocks. Amer. Geologist, 25 (May, 1900). Although .several statements in 

 this paper are hardly correct in the light of our present knowledge of the laws 

 governing the crystallization of heterogeneous systems, much of his explana- 

 tion of this texture appears to be substantially in accord with the most recent 

 views on this subject, but does not seem to have been referred to as it deserves. 

 Compare also a paper on the same subject by Professor L. V. Pirsson (Amer. 

 J. 8ci., 7 (April, 1889) whose views are discussed by Crosby in the second 

 paper referred to. 



