48 CALIFORNIA ACADEMY OF SCIENCES. 



dark basaltic rock with porphyritic augites and a sanidine 

 augite groundmass. The association of sanidine with 

 large amounts of augite is certainly a very rare occurrence, 

 although not altogether unknown. (Ponzatypus, Rosen- 

 busch, Micr. Phys. der Mass. Gest., p. 597.) 



It is true that these rocks do not appear as extravasated 

 masses, but as dikes; among them and associated with 

 them are many holocrystalHne and granular rocks ; but 

 among them are also glassy rocks and rocks with normal 

 trachytic structure ; and I feel confident that the name of 

 trachvic applied to them is more proper and suitable than 

 that of orthoclasc porphyry. 



It should 'be borne in mind that these trachytes, with 

 their great difference in structure and composition, all 

 occur within a quite limited district and that the pressure 

 under which they consolidated must have been practically 

 the same ; the rate of cooling, however, might have been 

 very different for the earlier and the later intrusions in the 

 same volcano, as Mr. Iddings and others have recently 

 pointed out. 



In the following pages a few of the different types will 

 be briefly described. 



a. Chiefly consisting of sanidine. This type is onl}' 

 represented by a 50' wide dike cutting cretaceous shales 

 in the southern foothifls a little west of the road across the 

 mountains to Fort Benton. A yellowish gray rock, some- 

 what porous and rough. Contains large thick tabular 

 phenocrysts of sanidine, i to 2 m. long, yellowish and 

 cracked. An alkali determination of the rock, made by 

 Dr. F. Gooch, gave Ka.^O : ii..82%, NagO : 2.5%. 77//;; 

 scc//on: Large, normal sanidine crystals in a trachytic 

 groundmass of feldspar microlites: much limonite ; isolated 

 biotite foils. The rock, when frc-sh. probably contained 

 more ferro-maunesian silicates. 



