CHARACTER AND DISTRIBUTION OF THE ROCKS 257 



quartz are present in specimens M, 50-16 (southeast 1/4 of southwest ^4:5 

 section 21, township 5 south, range 43 east), Henry Quadrangle, and 

 E. 2-16 (northeast 14 of southeast ^, section 24, township 7 south, 

 range 41 east). The rocks are rather fresh, except that the olivine is in 

 part altered to iddingsite, and secondary analcite is present in the vesicles 

 of some of the specimens. 



Specimens have been collected from the basaltic flows in many parts of 

 the district, a few of which are described below. Specimens M. 214-12 

 (southeast ^ of northeast 14, section 9, township 7 south, range 44 

 east) and E. 258-12 (southeast i/4 of southwest ^4? section 32, township 

 6 south, range 44 east), both from the Lanes Creek Quadrangle and 

 examined by Mr. Hunter, are aphanitic and vesicular basalts. In thin 

 section they are hypocrystalline, subophitic, and somewhat porphyritic, 

 showing a few scattered phenocrysts of plagioclase, olivine, and augite. 

 These are but little larger than the individuals of the ground-mass. The 

 ground-mass is composed chiefly of laths of plagioclase, irregular grains 

 of augite, olivine, magnetite, and a small amount of glass. The plagio- 

 clase has the approximate composition of labradorite. 



Specimens M. 33-16 (northeast ^4? section 25, township 4 south, range 

 40 east) and E. 95-16 (southeast i/4, section 13, township 2 south, range 

 40 east), both from the Cranes Flat Quadrangle and examined by Mr. 

 Larsen, are much alike. They are gray rocks with some small vesicles 

 and have the appearance of rather coarse diabases. Crystals of feldspar, 

 olivine, and augite are visible with a pocket lens. The microscope shows 

 that the rocks are diabasic in texture, and that they are made up of calcic 

 labradorite laths, olivine crystals, interstitial augite, and a little ground- 

 mass, apatite, and iron ore. The ground-mass is a glass filled with skele- 

 ton crystals. A small amount of analcite is in the vesicles. The olivine 

 is partly altered to iddingsite. 



All the basalts of the region here described apparently contain olivine 

 and may be classed as olivine basalts. There are, however, many differ- 

 ences in color and texture, particularly between the basalts that compose 

 the cones and those that constitute the flows. 



All are relatively fresh. Mechanical disintegration has occurred 

 locally, as indicated by blocky talus piles at the bases of many cliffs. 

 Chemical disintegration has produced little effect. The red color of the 

 scoria is probably in part at least original. The alteration of some of the 

 olivine to iddingsite and the development of analcite, as described above, 

 together with the formation of white, calcareous coatings in favorable 

 places, and the local development of brown weathered surfaces are the 

 principal chemical alterations to be noted. The soils above the basalt are 



