600 /. E. SPURR 



in crystallization from beginning to end. The rocks have evi- 

 dently crystallized entirely in their present position, and the slow 

 hardening which is ordinarily indicated shows that this point of 

 consolidation was originally some distance from the surface. 



In other specimens there have been slight breaks, bringing 

 about two, three, or more generations of crystals which in the 

 rocks near by are not distinguishable. These minor breaks were 

 due to slight migrations of material in certain bands, which 

 flowed slightly during the process of cooling, as is proved by the 

 angular fragments of finer-grained lava which they contain and 

 by the occasional broken condition of the phenocrysts. 



As explanation of the difference in crystallization between 

 the granular bands and the intercalated fine-grained ones, it 

 must be remembered that those bands which were affected by 

 flowage must have been at the time those possessing least 

 viscosity and consequently those which were least crystallized. 

 The final crystallization of these bands, therefore, took place 

 at a later period than that of granular bands, at which period 

 the rate of solidification was very likely more rapid. It is prob- 

 able, moreover, that the movement of flowage brought on of 

 itself a more rapid crystallization than if the rock had been 

 undisturbed, and that thus the finer-grained groundmass 

 orginated. The same suggestions hold good for the similar 

 phenomena, already described at Mason Butte. ^ 



' In connection with the conclusion above arrived at, i. e., that the phenocrysts 

 of the rocks were formed practically in place, compare the papers by Professor Pirs- 

 son and Professor Crosby. {Atn. Jour, of Sci., Vol. VIII, April, 1899, p. 271, "On 

 the Phenocrysts of Intrusive Igneous Rocks," and American Geologist, Vol. XXV> 

 J\o. 5, May, 1900, "On the Origin of Phenocrysts and the Development of Porphyritic 

 Texture in Igneous Rocks.") 



Professor Pirsson argues that the phenocrysts of intrusive rocks are not neces- 

 sarily intratelluric, and that there is no necessity of more than one period of crystalliza- 

 tion even for porphyritic rocks. From the fact that contact zones are often without 

 phenocrysts, while the rest of the rock contains them ; that in a contemporaneous 

 complex of dikes and sheets some may have phenocrysts while others do not ; from 

 observed cases where fluidal phenomena show that phenocrysts have developed after 

 the flowage ; from the arrangement of the crystals of the groundmass around some 

 phenocrysts, showing that these crystals have been crowded and shoved during the 

 growth of the larger crystals ; and from the fact that many granites (which have gen- 

 erally been considered intratelluric) contain very large phenocrysts, Professor Pirsson 



