CONTACT METAMORPHISM 261 



We may then summarize the succession of the lavas as follows : 



Older basalt ? 



Older andesite ? 



Older dacite ? 



Andesite ? 



Rhyolite. 



Andesite. 



Rhyolite. 



Alkali-rich andesite or latite. 



Piper Peak hypersthene-basalt. 



Dark olivine-basalt. 



Clayton Valley crater basalt. 



If we omit consideration of the older basalt, which apparently nowhere 

 was erupted in any considerable mass, we have a repetition of eruptions 

 of intermediate (andesite) and acid (rhyolitic and dacitic) magmas, 

 culminating with basic lavas (basalt). 



Contact Metamorphism 



pyroxenite 



The Paleozoic section north of Clayton valley has already been referred 

 to. The large area in which this section was measured contains numer- 

 ous dikes. They are in part acid lavas and in part diorite. At one point 

 on the northwest side of the area are small bodies of a green pyroxene 

 rock, along the contact of an intrusive hornblende-mica-gabbro. This 

 pyroxenite may have formed from the metamorphism of the adjoining 

 dolomite, inasmuch as the dolomite itself contains monoclinic pyroxene 

 in isolated grains and groups of grains. At another locality, at the con- 

 tact of limestone with a granite mass, another lime silicate mineral, 

 vesuvianite (see analysis below), formed, mixed with garnet and quartz. 

 Some of the slates along the contact were metamorphosed likewise into 

 schists. The pyroxene in the dolomite and the vesuvianite appear to be 

 contact metamorphic products formed by recrystallization due to the 

 heat and vapors of the intrusive granite rocks. 



Analysis of Vesuvianite, Specimen Number 186 25 

 George Steiger, Analyst 



Si0 2 36.80 



TiO, 0.66 



A1 2 3 17.53 



Fe 2 3 1 .56 



FeO 3.27 



MnO 0.48 



MgO 1.23 



CaO 35.00 



