420 DEPARTMENT OF THE INTERIOR 



2 GEORGE V., A. 1912 

 Order of Eruption of the Midway Lavas. 



A summary of the facts actually determined for the relations of the 

 r rarious lavas of the Midway group may he given in a few words. All are clearly 

 jomnger than the older Kettle River Oligocene beds but it is quite possible that 

 the basalt and augite andesite, if not much of the biotite-bearing andesite, were 

 contemporaneous with the younger sandstones. All of the andesites and basalts, 

 apparently without exception, are diked by the trachyte or its equivalent, the 

 ■ pulaskite porphyry. The relation of the trachyte to the extrusive rhomb- 

 porphyry and the analcitic lava is not quite clear. In the section east of the 

 Kettle river bridge a dike of the rhomb-porphyry, as shown in Figure 25, cuts a 

 ^ great sill of the pulaskite porphyry. At two other points the relation seems to 

 lie reversed, though the field evidences are not there so compelling as in the 

 first mentioned case. The flows of trachyte, rhomb-porphyry, and* analcitic lava 

 (shackanite) are closely associated and are in apparent alternation. All three 

 types seem to belong to one eruptive period which probably opened with the 

 extrusion of trachyte and closed with the analcitic rhomb-porphyry (shackanite). 



The probable succession of the lavas is, then, as follows : — 



Lavas. Corresponding intrnsives 



of region. 



C Analcitic lava (shackanite) 1 pu u u 



"Youngest group. { Extrusive rhomb-porphyry J "bomb-porphyry. 



(.Alkaline trachyte Pulaskite porphyry. 



(Biotite andesite "J 

 Biotite-augite andesite I . •. , . , .. , 



Hornblende-augite-biotite andesite Uugite-biotite porphyrite 

 Hornblende-augite andesite J 



,-t, , . f Augite andesite Augite porphyrite. 



'Oldest group (olivine basalt Augite gabbro. 



The oldest and middle groups of the lavas are believed to be of Oligocene 

 age. The youngest group may conceivably belong to the late Oligocene but it is 

 more probable that these peculiar lavas together with the chonoliths, dikes, and 

 sills of rhomb-porphyry and pulaskite porphyry, were erupted during or just after 

 the deformation of the Oligocene Kettle River beds. Similar deformation of older 

 Tertiary strata elsewhere in British Columbia and in Washington State has 

 been credited to the close of the Miocene period. The rhomb-porphyries and the 

 trachyte are provisionally referred to that stage of geological history. 

 On this view the Midway volcanic group is a compound formation involving 

 products of two distinct volcanic epochs in this region. 



Structural Relations of the Columbia Mountain System west of 



Christina Lake. 



In its complexity the group of mountains discussed in this and the preced- 

 ing chapter is much like the Rossland mountain-group. The western group 

 has, however, an extra series of volcanic and sedimentary rocks of Tertiary age, 

 which are almost certainly not represented at any point in the Rossland 

 mountains. 



