210 DEPARTMENT OF THE INTERIOR 



2 GEORGE V., A. 1912 



The rock of zone d (150 feet thick) shows an occasional large pbenocryst 

 of labradorite, but usually it is a blackish green, compact, homogeneous mass, 

 bearing numerous amygdules of all sizes up to '8 centimetres in length. The 

 amygdules, often orientated roughly parallel to the surface of the lava flow, 

 are composed of infiltrated quartz or chlorite, or both; sometimes green biotite 

 replaces some of the chlorite. In thin section the only original constituents 

 are octahedra of magnetite and a plagioclase. The latter in individuals rang- 

 ing from 0-5 mm. or less to 1 mm. in length, has a maximum extinction of 19° 

 or 20° and seems to be acid labradorite, as in the underlying rocks. The char- 

 acteristic arrangement of the abundant plagioclase crystals is that of a typical 

 diabase. The interspaces are entirely filled with pale green chlorite and the 

 original grains of magnetite, along with leucoxene and a little limonite. It 

 seems impossible to say whether the chlorite has been derived from a pyroxene 

 or from a glass. The habit of the rock is that of an ordinary ba3alt. Its 

 specific gravity varies from 2-909 to 3-078; the average of three specimens is 

 3-000. 



In zone d numerous, though small, angular fragments of quartzite and 

 metamorphosed argillite, studded with numerous conspicuous octahedra of 

 magnetite, were observed. 



At the top of zone d is a conformable bed of argillite a few inches thick. 

 The lava (65 feet thick) overlying this sediment belongs to a second period of 

 extrusion closely following the former one. 



In view of all the facts it seems certain that the whole 465 feet of lava 

 represented in the section represent a single chemical type. It is highly pro- 

 bable that the lower 400 feet belong to one great flow and that the high vesicul- 

 arity of zone d, the conspicuously porphyritic character of zone c, and the 

 special features of zones a and h are all the results of different conditions of 

 cooling in that thick flow. 



The columnar section of the formation in this section is. therefore, as 

 follows : 



Top, conformable base of Gateway (Moyie) formation. 



Second lava-flow : /., 65 feet — amygdaloidal lava poor in phenocrysts. 



Inter-bed: e. 4, inches — argillite. 



/ d. 150 feet — amygdaloidal lava poor in phenocrysts. 



Wirot l»9fl™. / c - 20 ° " highly porphyritic, non-vesicular lava, 

 inrst lava-flow. b 1Q „ compact lava> 



lo. 40 " brecciated ' aa ' lava. 



Total lava 465 feet. 



Base, conformable top of Kitchener (Siyeh) formation. 



On examining the sections of the formation farther east it was foimd 

 that the four lava phases just described were not regularly represented. On 

 the summit twelve miles west of Gateway the striking porphyritic phase is 

 almost entirely replaced by the amygdaloid of zone d in the' type section, while 



