VARIATIONS OF TEXTURE IN IGNEOUS ROCKS 6or 



TRANSITIONS OF TEXTURE IN THE GRANITE-RHYOLITES OF THE 

 QUINN CANYON RANGE. 



Field description. — The Ouinn Canyon range lies a long dis- 

 tance east ot all the other localities which have been described, 

 being almost due south from Eureka and nearly west of Pioche. 

 The whole southern portion of the range is buried in rhyolitic 

 flows. "^ The range was examined by the writer at its northern 

 end, where the Paleozoic core of the mountains emerge from the 

 volcanic covering. On the western side of the range, near the 

 contact of the Paleozoics with the rhyolite, the stratified rocks 

 are pierced by numerous great dikes, which vary from coarse to 

 fine in texture. These dike rocks seem similar in composition, 

 and sometimes in texture, to the rather massive rhyolite which 

 forms the hills to the west of this locality. 



Microscopic description. — A specimen of the main rhyolite 

 examined under the microscope has the following characteristics : 



Rhyolite (241 N.) . — Tliis rock has phenocrysts of all sizes, 



reasons that phenocrysts are not necessarily of a distinct crystallization period as 

 compared with the groundmass, but may in some cases be formed in place. Professor 

 Pirsson advances the explanation that a comparatively rapid fall of temperature and 

 decrease of hydration, resulting in a viscosity augmenting in an increasing ratio, may 

 produce monogenetic phenocrysts (that is, phenocrysts which occur only in a single 

 generation). Recurrent phenocrysts (that is, those occurring in more than one gen- 

 eration) he explains as due to mass action, believing that minerals which are present 

 in very large quantity are especially active cryslallizers. 



Professor Crosby believes that no sudden changes of temperature, hydration, or 

 pressure, are necessary for the formation of phenocrysts. He believes that in a gradu- 

 ally consolidating rock the crystallization first established may be brought to a close 

 by the gradually increasing viscosity and that after passing this critical point new 

 zones of crystallization will be established, of much smaller field, and at this point 

 the groundmass begins and the phenocrysts end. If the rate of cooling is still 

 slower, an allotriomorphic granular texture results, while if the rate is more rapid the 

 texture may be glassy or nearly so. 



The deductions of the writer, given above, agree with those of the authors cited 

 in this, that phenocrysts may be formed in place. His observations, however, go to 

 show that where cooling is strictly uniform there will be no distinct generations, but 

 a gradual transition from phenocrysts to groundmass ; whereas, if there are distinct 

 generations, they are brought about by breaks in the conditions of consolidation, even 

 though these breaks be comparatively slight. 



' G. K. Gilbert, Survey West of the looth Meridian; Vol. Ill, Geology, p. 122. 



