1 78 EDSON S. BASTIN 



portions of the groundmass in any other way than that described 

 above. Of the minerals of the groundmass, apatite seems to have 

 been the first to crystalHze, followed by titaniferous magnetite, 

 biotite, quartz, and hornblende. In only a few instances is the 

 hornblende inclosed in biotite, but the reverse relation is very 

 common. 



Near the border of the area the porphyry has, in many places, 

 been considerably sheared. The feldspars for the most part remain 

 intact, though showing some fracturing and bending under the 

 microscope. They have become coated, however, with a "skin" 

 of secondary biotite which adheres even after they have weathered 

 out from their matrix (Fig. 2). Except for this development of 

 mica, the sheared rock differs but little in microscopic appearance 

 from the unsheared. 



The chemical analysis, and the classification of this rock accord- 

 ing to the quantitative system, are given in the tables on the opposite 

 page. 



According to Washington's^ table of rock analyses, the only 

 other known rock falling in this subrang is one collected by G. K. 

 Gilbert from Two Buttes, Prowers County, Colo., and briefly des- 

 cribed by Whitman Cross. ^ This is a laccoHtic rock, and has as 

 its chief constituents diopside, alkali feldspar, considerable biotite 

 magnetite, and olivine. The ferromagnesian minerals predom- 

 inate. 



These rocks are peculiar in their high alkali content and in the 

 preponderance of potash over soda. The only other highly alka- 

 line rocks (perkalkalic or domalkalic) of this class which show a 

 corresponding preponderance of potash (perpotassic or dopotassic) 

 are wyomingose, madupose, and orendose from the Leucite Hills 

 of Wyoming, chotose from the Bearpaw Mountains of Montana, 

 and albanose from the Alban Hills of Italy. All of these rocks 

 show leucite in the mode, and in all except orendose it is present 

 also in the norm. In orendose the high potash content is indicated 

 by the presence of potassium metasilicate in the norm. In prower- 



^ H. S. Washington, "Chemical Analyses of Igneous Rocks," Professional Paper 

 No. 14, U. S. Geological Survey, p. 313. 



2 See Bulletin No. 228, U. S. Geological Survey, p. 186. 



