604 /. E. SPURR 



composition. Another interesting conclusion is that the change 

 between the lava texture and the granitoid texture consists 

 chiefly in the coarsening of the groundmass. 



In 1899, Messrs. Tower and Smith described textural transi- 

 tions in the Tintic range in Utah, which lies within the petro- 

 graphic province of the Great Basin and is situated southward 

 from Great Salt Lake.' In this district is found pyroxene 

 andesite or perhaps more properly latite, which is an effusive 

 rock and is closely associated with granular monzonite. There 

 are all variations of texture between andesite with glassy ground- 

 mass to that with a holocrystalline groundmass ; from this to 

 closely similar rocks, also with holocrystalline groundmass, which 

 are called monzonite porphyry ; and from these through panidio- 

 morphic granular phases to those of hypidiomorphic granular 

 structure. 



CONCLUSIONS. 



In the Great Basin, particularly in Nevada, we have Tertiary 

 extrusive rocks which show transitions from a granular structure 

 with glassy groundmass. The different phases are often inti- 

 mately associated, and structural analysis shows that the differ- 

 ences of crystallization which brought about these variations 

 were slight, a relatively small decrease of the rate of cooling 

 being sufficient to allow the formation of the holocrystalline 

 instead of the porphyritic structure. 



Transitions similar to those found in the Great Basm have 

 been sparingly chronicled elsewhere. These appear to become 

 rare in proportion as the rocks become siliceous. This is so 

 because with a given relatively rapid rate of cooling a magma 

 of basic composition will consolidate with a holocrystalline 

 structure, while a siliceous magma will become fine-grained 

 and porphyritic. We have accordingly many instances of 

 holocrystalline diabases which are certainly extrusive, and of 

 similar rocks in rather fresher condition (generally due to 

 their being younger) which have been called dolerites. In 

 the more siliceous rocks such textural transitions are rare in 



^Nineteenth Ann. Rept. U. S. Geol. Surv., p. 656. 



