REPORT OF THE CHIEF ASTRONOMER 315 



SESSIONAL PAPER No. 25a 



300 yards south of the Boundary slash. The acid dike is a typical biotite 

 granite porphyry. It is between 200 and 300 feet wide and is paralleled by other 

 great dikes of similar material outcropping at low water in the islets of the 

 river channel. They may be acid apophyses from the extensive Trail batholith 

 toward which they strike; they are, however, noted here because their relation 

 to the younger minettes is very clear. (See Figure 23.) 



A white aplitic sill cutting the Pend D'Oreille phyllitic schist on the right 

 bank slope of the South Fork of the Salmon, about 2-'5 miles S. 30° W. of the 

 summit of Lost mountain, may be mentioned on account of the unusual struc- 

 ture of the rock. It is slightly porphyritic with phenocrysts of quartz and 

 sodiferous orthoclase. The ground-mass is partly the common panidiomorphic 

 aggregate of quartz and alkaline feldspar (much sericitized) but contains 

 quite numerous, small spherulites of alkaline feldspar which is developed in 

 rosettes. A few grains of magnetite represent the only other constituent. 

 The relations of this sill to the other granitic rocks of the range are unknown. 



Dike Phases of the Rossland and Beaver Mountain Volcanics. 



The formations older than the Rossland and Beaver Mountain lavas are, 

 naturally, cut by dikes which indicate vents for the lavas or the fillings of 

 fissures connected with those vents. A few of these dikes have been found in 

 localities where erosion has stripped away the volcanic cover and some of them 

 have been microscopically examined. Among these, four types may be listed 

 but it should be understood that the list does not exhaust the different varieties 

 of the dikes genetically connected with the volcanics. 



Just east of the large boss of Sheppard granite mapped on the Pend 

 D'Oreille river, the schists are traversed by a fifty-foot, nearly vertical, north- 

 south dike of porphyritic monzonite. The phenocrysts are stout prisms of 

 augite up to 8 mm. in length. The essentials of the hypidiomorphic-granular 

 ground-mass are orthoclase, microperthite, labradorite (Ab 3 An 4 ), augite and 

 biotite; the essentials are magnetite, apatite, zircon and a little interstitial 

 quartz. The plagioclase crystals are characteristically clumped in the ortho- 

 clase mesostasis. 



About three-quarters of a mile north of Old Fort Sheppard, where the 

 mountain-spur projects through the terrace sands and gravels to the Columbia 

 river, there are large outcrops of slaty and quartzitic rock which have been 

 mapped as part of the Pend D'Oreille group. The crumpled and mashed slate 

 is here cut by a 25-foot vertical dike of dark-gray hornblende-biotite monzonite 

 striking N. 8° E. (visible at low water). Some 300 yards south of the Boundary 

 slash on the same side of the river and at the water's edge, three dikes from 

 ten to thirty-five feet wide and of macroscopic appearance somewhat similar 

 to the monzonite were found to consist of hornblende-augite gabbro. In this 

 type the feldspar is basic labradorite (Ab„ An 3 ), and alkaline feldspar is entirely 

 absent; a few foils of biotite are accessory. 



