REPORT OF TEE CHIEF ASTRONOMER 417 



SESSIONAL PAPER No. 25a 



rite bearing phenocrystic labradorite and dark-green hornblende, with a felds- 

 patbic base in which a little primary quartz may be discovered under the 

 microscope. 



The augite-biotite porphyrite forms a number of great dikes and a large 

 chonolith-like mass cutting the Paleozoic limestone and quartzite on Deer 

 Hill. An apophysal sill from the largest body cuts the tilted Kettle River 

 sandstones at the western base of the hill. The same intrusive appears at 

 several other places in the Boundary belt but the exposures were often not full 

 enough to declare the structural relations of the bodies. In some cases it was 

 not possible to tell if the rock were really not a non-vesicular phase of the 

 surface lava. The Deer Hill dikes and chonolith (?) are mineralogically like 

 the biotite-augite andesite of the flows, with the natural exception that the 

 porphyrite is more thoroughly crystallized. The ground-mass is here holocrys- 

 talline and generally carries a notable amount of free quartz along with the 

 feldspar, augite, and biotite microlites. The porphyrite of the apophysal sill 

 mentioned carries a little phenocrystic hornblende as well as the biotite and 

 augite. 



]STo dikes corresponding to the effusive olivine basalt are known to occur 

 in the belt, though it is quite possible that some of the observed basalt is really 

 in dike relation. A 300-foot sill of fine-grained augite gabbro cuts the Ter- 

 tiary sandstone on the southern face of the conspicuous north-south ridge 

 three miles due east of the mouth of Rock creek. This gabbro is probably an 

 intrusive phase of the same magma which is represented in the vesicular 

 basaltic flows across the river. The rock has the mineralogical composition 

 of a typical, fine-grained diabase but has the hypidiomorphic-granular structure. 

 The pyroxene is common augite without the diallagic parting. A few serpen- 

 tinized grains of olivine appear in one thin section, suggesting a transition 

 to the olivine gabbros. A few sporadic flakes of biotite are accessory in the 

 same thin section. In the sill-rock generally the only essentials are augite and 

 labradorite. 



Pulashite Porphyry. — In the stretch of eight miles between Ingram creek 

 and the mouth of Rock creek the Tertiary sediments are cut by a large number 

 of thick sills and dikes of a pulaskite porphyry, which is in strong lithological 

 contrast to all the other igneous types of the region except the alkaline trachyte. 

 The porphyry is conspicuous in the field and, if the exposures were sufficient, 

 it could be mapped with relative ease. Most of the intrusions were found on the 

 north side of the Kettle river, but a few dikes of the porphyry cut the andesites 

 on the south side. 



Just east of the bridge six miles above Midway the porphyry is specially 

 developed, generally as sills in the fossiliferous Tertiary sandstone. These sills 

 vary from 100 feet or less to about 800 feet in thickness. Toward the top of the 

 broad hill east of the bridge and 1,000 feet above the river, the sandstones are 

 squarely truncated by an apparently continuous mass of the porphyry of which 

 the silk are offshoots. This larger mass has been injected into the sandstones 

 without any definite relation to bedding-planes and is perhaps best described as 



25a— vol. ii— 27 



