636 THE JOURNAL OE GEOLOGY. 



uents are orthoclase, albite and hornblende. The relative proportions 

 of the minerals has been estimated to be: orthoclase, 0.50; albite, 

 0.16; hornblende, 0.23; sodalite, 0.08; analcite, 0.03. The horn- 

 blende was analyzed and found to correspond to barkevikite. Mr. 

 Lindgren calls attention to the resemblance in chemical composition 

 between this rock and many nepheline-syenites, except for the rela- 

 tively higher percentage of K2O in the rock from Square Butte. He 

 also notices the striking similarity between the analysis of this rock 

 and those of certain leucitophyres from Rocca Monfina, and remarks 

 that under different conditions the same magma, now crystallizing as 

 a sodalite-syenite, might have produced a leucite-feldspar rock. 



Trachytic rocks, with a great variety of habits, are abundant in 

 the Highwood mountains. The essential minerals are sanidine and 

 augite, with less prominent biotite. The augite is deep green, often 

 somewhat pleochroic, and evidently contains an admixture of the 

 segirine molecule. It is very characteristic not only of the trachytes 

 but also of the basaltic dike rocks of this region. These rocks form a 

 connected series, the members of which differ in the relative quanti- 

 ties of augite and sanidine composing them. At one end of the 

 series is a rock consisting almost wholly of feldspar, and at the other 

 end a dark basaltic rock with porphyritical augites and a groundmass 

 of sanidine and augite. In structure these rocks range from holocrys- 

 talline and granular to glassy. Some of the trachytes contain small 

 crystals of sodalite (?) inclosed in sanidine. In one form of the rocks 

 sanidine ceases to be the prominent phenocrysts and augite takes its 

 place, and olivine occurs in the groundmass, which consists of feld- 

 spar and colorless glass easily soluble in HO. Associated with the 

 sodalite-syenite of Square Butte are dark colored basaltic rocks, which 

 occur in three sheets at the base of the butte. Surrounding the butte 

 there are numerous dikes apparently radiating from the central mass. 

 One of these basaltic sheets contains phenocrysts of augite, olivine, 

 brown mica, and white isometric crystals whose original character is 

 uncertain. The rock is considerably decomposed. Another of the 

 sheets is like analcite-basalt but is also decomposed. The third is 

 coarsely granular and approaches theralite in composition. 



The rocks described as analcite-basalts occur in dikes and possibly 

 as necks in association with the rocks already described. They con- 

 sist of augite, olivine, magnetite, and a mineral, which from its form 

 and optical properties, and from its chemical composition appears to 



