REPORT OF TEE CHIEF ASTRONOMER 145 



SESSIONAL PAPER No. 25a 



members and could scarcely escape detection if they were repeated in the 

 various sections, especially in the one tra.versed ' on the nearly treeless ridge 

 south of Monk creek. The total thickness is, then, taken to be at least 6,000 

 feet; it may be 7,000 feet or more. 



The rocks composing the Irene Volcanic formation, as it crops out in the 

 Boundary belt have been grouped in divisions as here shown : — 



Columnar section of Irene Volcanic formation. 



Top, conformable base of Monk formation. 



50 feet. — Greenstone schist, a crushed basic amygdaloid. 

 200 " Angular conglomerate or breccia with phyllitic cement. 

 1,710 " Greenstone schist with a few thin bands of phyllite toward the top. 



40 " Gray to white, fine-grained dolomite. 

 4.000-f-" Sheared and greatly altered basaltic and andesitic lavas = largely greenstone 

 schist. 



6,000±feet. 



Base, conformable top of Irene Volcanic formation. 



The great bulk of the formation is composed of a notably uniform type 

 of highly altered andesitic lava, now typical greenstone. It is a dark green 

 or greenish gray, compact, schistose rock, in which, as a rule, there is scarcely 

 a trace of the minerals originally crystallized out of the magma. A large pro- 

 portion of the greenstone is amygdaloidal, the amygdules (composed of calcite 

 or, much more rarely, of quartz) being mashed out into thin lenses parallel to 

 the pronounced schistosity. While the greenstone has been essentially derived 

 from surface lava flows, it is usually impossible to distinguish the limits of 

 any one flow. The difficulty of doing this is evidently due in part to the 

 intense mashing and metamorphism of the lavas. It appears probable that, 

 while the great mass was accumulated by many successive flows, each flow was 

 of considerable thickness. 



From the study of over twenty-five thin sections cut from as many typical 

 and relatively unweathered specimens, it has been found that throughout the 

 entire thickness, the rock has a very homogeneous character. It is a confused, 

 felted mass of uralite, chlorite, epidote, quartz, calcite, limonite, sericite, saus- 

 surite, and often biotite, with which pyrite, magnetite, and ilmenite (generally 

 altered to leucoxene) regularly form accessories in variable amount. 



For several thin sections this list exhausts the list of constituents; in 

 their corresponding rocks metamorphism has evidently been thorough. 



Excepting possibly the iron ores, the only original magmatic constituent 

 is plagioclase, which with surprising regularity is represented in most of the 

 sections only by a few, highly altered, broken crystals. The form and relations 

 of these crystals show that they generally formed phenocrysts in the original 

 lava, which had an abundant glassy or macrocrystalline base. An exceptional 

 holocrystalline, ophitic, fine-grained phase was found near the base of the 

 formation on the ridge just north of the Boundary line. In two thin section- 

 of this phase the plagioclase is better preserved and gave in tke zone of 

 25a — vol. ii — 10 



