REPORT OF TEE CHIEF ASTRONOMER 541 



SESSIONAL PAPER No. 25a 



stock and most of these dikes have the composition and structure of a quartz- 

 bearing monzonite verging on granodiorite. One great dike has the properties 

 of a typical hornblende-diorite porphj r ry. 



Monzonite Stock. — The stock is intrusive into the volcanics at their fault- 

 contact with the Hozomeen quartzites a short distance north of Monument 69. 

 This stock as exposed has an elliptical ground-plan, measuring about 1,200 

 yards in its greatest diameter and 800 yards along the minor axis. Like the 

 agglomerate it is devoid of any crush-schistosity and the intrusion appears to 

 have occurred later than the post-Laramie epoch of intense crushing. The 

 intrusion may have been genetically connected with the faulting by which the 

 volcanics were dropped down into their present lateral contact with the old 

 quartzites. 



The material of the stock seems to be rather uniform, with the habit of a 

 fresh, light gray, medium-grained syenite. The essential constituents are, in 

 the order of decreasing importance : plagioclase, orthoclase, hornblende, quartz, 

 biotite, augite. The plagioclase is often zoned, with Afy An t in the cores and 

 oligoclase in the outer shells ; the average mixture is an andesine near Ah_ An.. 

 The hornblende is green in about the same tones as those of the amphibole in 

 the Chilliwack granodiorite. The characters of the other essential minerals 

 and of the accessories are also the same as in that batholith. The structure is 

 the eugranitic. 



The rock clearly belongs among the quartz-bearing monzonites and chemi- 

 cally would show the composition also allied to that of a basic granodiorite. 

 The Chilliwack batholith is only five miles distant and it is highly probable 

 that this monzonite stock is its satellite. 



Dikes. — On the rugged, glacier-covered ridge south of Monument 68, the 

 Skagit volcanics are cut by two or more great, north-and-south dikes of mon- 

 zonite, similar to the staple material of the stock but relatively richer in 

 plagioclase and quartz and poorer in biotite. These dikes, which range from 

 100 to 300 feet or more in width, are doubtless giant apophyses from the magma- 

 chamber of which the Chilliwack batholith was a part. 



A 'half-mile west of these dikes and running nearly parallel to them is a 

 third dike over 100 feet in width. It is composed of a dark gray to greenish- - 

 gray, medium to fine-grained, somewhat porphyritic rock of different habit from 

 the monzonite. The phenocrysts are green hornblende which is often in parallel 

 intergrowth with augite; and basic labradorite. The ground-mass is a hypi- 

 diomorphic-granular aggregate of labradorite and interstitial quartz. Magnetite, 

 apatite, and titanite are the accessories. Orthoclase seems to be absent. The 

 rock is somewhat altered and is charged with a considerable amount of uralite 

 evidently derived from augite. A small amount of chlorite may represent 

 original biotite, but none of this mineral was discovered in the thin section. - 

 The rock is to be classed as a hornblende-diorite porphyrite. 



The dike is uncrushed. It has the habit and nearly the composition of the 

 finer-grained phases of the Slesse diorite. The similarity is so great that one 



