BY H. I. JENSEN. 85 



in Kelly's paddock, near Ivory Knob. Much of this rock is 

 beautifully spherulitic. 



Microscopically this rock consists of an isotropic glass, which 

 shows beautiful perlitic structure. A few phenocrysts of ortho- 

 clase occur in it. These often include albite lamella3 in parallel 

 intergrowth, and sometimes zircon. 



Name : Perlitic Vitrophyric Trachyte, or Perlitic Trachyte- 

 obsidian. 



The SiOo percentage was determined, and found to be 69*74, 

 so that this glass probably has the pantellarite composition. 



Fl. 11. Handspecimen : a reddish, aphanitic, banded rock 

 showing flow-structure; very like Fl.l, and F1.2. 



Microscopic texture : hiatal-porphyritic phenocrysts are seen 

 in a microcrystalline to microspherulitic base. The phenocrysts 

 consist of anorthoclase ; felspar also forms stellate groups of 

 crystallites in the spherulitic base. Most of the base appears to 

 have the composition of felspar. Yellowish globulites, a few 

 minute green segirite fragments, some magnetite grains, and 

 ilmenite plates, and a few xenogenic biotite- and quartz-fragments 

 constitute the accessories, which all together form less than 2 % 

 of the rock. 



The silica percentage was found to be 72 %, hence the base 

 must contain quartz. 



Name : Microspherulitic Qnartz-bearing Soda-Rhyolite, allied 

 to Pantellarite. 



F1.9. Loc. : Summit of Mt. Blaine. 



Handspecimen : white, megascopically porphyritic trachyte. 



Texture — Crystallinity : holocr3^stalline. Grain-size : uneven, 

 with serial porphyritic phenocrysts in a microcrystalline base; 

 dopatic. Fabric : trachytic to orthopliyric. 



Composition: the main constituent is felspar. The phenocrysts 

 vary in size from mediophyric to mediiphyric. Their refractive 

 index is always less than that of Canada balsam. Carlsbad 

 twinning is common, but, in addition, we have a shadowj'^ extinc- 

 tion due to the interlamellation of two different felspars, and to 



