416 DEPARTMENT OF THE INTERIOR 



2 GEORGE V., A. 1912 



an east-west distance of three miles, has excellent bed-rock exposures which 

 afford some indication of the mode of occurrence of the lava. On the mountain 

 due north of the bridge it is seen to form massive flows, each a hundred feet 

 or more in thickness. These are conformably interbedded with several equally- 

 heavy flows (100-200 feet thick) of a vesicular trachyte of quite different habit 

 and composition. The two lavas vary in their power of resistance to weather- 

 ing; the great beds strike regularly N. 45° E. and dip at an average angle of 

 30° to the southeast. As a result the mountain is strongly ribbed with alternat- 

 ing scarps and back-slopes with crests of the corresponding ridges trending 

 N.E.— S.W. (See Plate 73, B.) 



On a specially sharp, meridional, 3,100-foot ridge, situated 2-5 miles 

 farther west, the analcitic lava with typical character was observed resting on 

 the Tertiary sandstone. At this point the strike is nearly due north and south; 

 the dip, 35° E. It is most probable that, between the two localities, the lavas 

 and sandstone are repeated in outcrop by a number of strike-faults. The 

 sandstones are cut by large dikes of the rhomb-porphyry with the habit of the 

 chilled phase of the chonolith. These dikes were probably among the feeders 

 of the lava flows. 



The analcitic porphyry is practically identical in character with that des- 

 cribed on the west side of the Kettle river. It is here somewhat more vesicular, 

 the flattened pores reaching an inch in greatest diameter. They are filled 

 chiefly with calcite but a very few carry a yellowish zeolite. 



Intrusive Eocks cutting Kettle Eiver Strata. 



All the nine types of the Tertiary lavas were necessarily erupted through 

 fissures or other vents in which the chemically corresponding ' hypabyssal ' 

 rocks have crystallized. Most of the lavas are, in fact, paralleled in the dike 

 and sill rocks which at many points within the Boundary belt cut the Tertiary 

 sediments and the Paleozoic formations. 



Porphyrias,. — It has been noted that most of the andesitic species are 

 closely similar in their chemical composition; their distinction as species is 

 chiefly based on mineralogical characters which are doubtless due in largest 

 part to differences of physical conditions during crystallization. Where the 

 same magmas were intrusive, temperature and some other conditions must 

 have been more uniform than were those prevailing during the solidification 

 of the surface lavas. This is a probable reason why few distinct types have 

 been found' among the dikes or sills corresponding to the Midway andesitic 

 flows. 



These types are two in number — hornblende porphyrite and augite-biotite 

 porphyrite. The former composes a poorly exposed sill cutting the Tertiary 

 shales and sandstones at the gulch which mouths a half mile northwest of the 

 Canadian Pacific railway ' Y ' at Midway. This is a normal fine-grained porphy- 



