REPORT OF THE CHIEF ASTROS OVER 331 



SESSIONAL PAPER No. 25a 



practically impossible to determine the character of the original ground-mass, so 

 great has been the alteration of the rock. The other secondary products, as well 

 as the accessory minerals are like those in the augite-biotite latite, to -uhich the 

 hornblende-biotite latite must be chemically quite similar. 



Biotite Latite. — A sixth type represents a lava which is macroscopically like 

 a mica andesite, but under the microscope shows features relating it to the latites 

 just described. In its present condition it is a greenish-gray rock of decidedly 

 lighter tint than the great majority of the Bossland lavas. Biotite and labra- 

 dorite (averaging about Ab t AnJ are the only phenocrysts. The base was 

 probably once largely glass in which microlites of labradorite and more irregular 

 ones of (probably) orthoclase were embedded. The ground-mass is now 

 abundantly charged with secondary sericitic mica and some quartz which is 

 doubtless also of secondary origin. 



This rock has not been chemically analyzed but the analysis of a fresh speci- 

 men would correspond to many mica andesites which are rich in potash. In 

 view of the intimate association of this type with the undoubted, analyzed latites, 

 it seems best to regard the rock as a salic latite rather than a true andesite, 

 though it must be on the border-line between the two species. 



Femic Augite Latite. — Finally, an altered lava which seems to represent an 

 opposite pole in the differentiation of the latitic magma, was found on the 

 eastern slope of the hog-back ridge in (the western) Sheep creek valley. A 

 second but more doubtful occurrence was noted on the ridge between Boundary 

 monument No. 170 and the Coryell batholith. The chief mineralogical differ- 

 ence between this type and the analyzed augite latite consists in a great 

 increase in the number of augite phenocrysts, a corresponding decrease in the 

 abundance of labradorite phenocrysts (which may entirely fail in the thin 

 section), and apparently a decrease in the relative amount of the ground-mass. 

 The accessory and secondary minerals are the same as those noted for the 

 augite latite; orthoclase was not observed in the ground-mass, which in all 

 the collected specimens has largely gone over to green biotite and sericitic mica. 



Comparison with Sierra Nevada Latite and with Average Monzonite. — 

 Before noting the other types of lava in the Bossland group it will be instruc- 

 tive to review their classification in terms of the chemical constitution of the 

 original latites as defined by Bansome. In Table XXII., Col. 1, the average of 

 the Bossland latites is given, and in Col. 2 the average of six typical latites 

 from California. Col. 3 shows the average of all ten latites and Col. 4, the 

 average of the twelve monzonites recorded in Osann's compilation of chemical 

 analyses throughout the world. The last three averages have been reduced to 

 100 per cent. In making the average of the Bossland latites the augite latite 

 and augite-biotite latite were considered as of equal weight and their average 

 was weighted as four against the average of the analyses of the hornblende- 

 augite latite and olivine-augite latite which together were weighted as unity. 

 Thi3 weighting corresponds approximately to the relative volumetric impor- 

 tance of the different types in the Bossland district. 



