REPORT OF THE CHIEF ASTRONOMER 143 



SESSIONAL PAPER No. 25a 



The cement of the conglomerate is usually in large amount and rather 

 uniform throughout the formation. Originally it must have been of the nature 

 of a graywacke or very muddy sand. In its present condition it is a schistose, 

 crystalline mass of various shades in gray and greenish-gray. Clastic grains 

 of quartz of all sizes up to one or two millimetres in diameter, and very much 

 rarer grains of orthoclase lie embedded in an extremely abundant fine-grained 

 matrix of sericitic muscovite, biotite, and chlorite. The foils of mica are 

 specially developed in the planes of schistosity. Grains of magnetite, leucoxene 

 and pyrite are constant subordinate accessories, while anhedra and minute 

 idiomorphic crystals of titanite are often very abundant in thin sections. 

 Irregular or roughly rhombohedral, secondary crystals of calcium carbonate 

 (probably somewhat magnesian), a millimetre or less in diameter, seldom fail 

 to appear in the sections. They sometimes, though not always, enclose quartz 

 and the micas poikilitically. Quite often the clastic quartz grains show the 

 familiar proofs of secondary enlargement. 



The mass of the conglomerate may be interrupted by lenses of metamor- 

 phosed sandstones and pelites a few inches to several feet in thickness. These 

 rocks have been metamorphosed to phyllitic schists of composition practically 

 identical with that of the conglomerate cement. 



The specific gravity of five type specimens of the conglomerate ranges from 

 2-680 to 2-753. Their average, 2-732, is believed to be nearly the average for 

 the whole, fairly homogeneous formation. 



After field and laboratory study of these rocks there can be little doubt 

 as to the origin of some of the clastic materials. The colour, composition, 

 and general field habit of the quartzite, phyllite, and dolomite pebbles clearly 

 show their derivation from the underlying Priest River terrane. Nevertheless, 

 the writer has not found a single - pebble of the spangled quartz -mica schists so 

 abundant in that terrane and, in general, the larger quartzite pebbles show a 

 massiveness or lack of schistosity, which is more marked than that expected 

 if they were derived from the Priest River terrane in its present lithological 

 condition. It seems necessary to conclude that a large proportion of the meta- 

 morphism suffered by the older terrane, including the growth of the biotite 

 spangles and some of the intense shearing and sericitization of the quartzites. 

 has affected the terrane since the Irene conglomerate was rolled on the ancient 

 beaches. One may naturally hold that the metamorphisni of the Priest River 

 terrane occurred simultaneously with the mashing and partial recrystallization 

 of the Irene conglomerate as younger and older formations were upturned 

 together. Even in the conglomerate there is striking proof of immense tangen- 

 tial pressure and crushing such as is nowhere given in the Purcell, Galton, or 

 Lewis series of formations. 



Since most of the material for the conglomerate was won from the older 

 terrane, which in this region is not known to contain pre-Irene acid 

 plutonic masses on any large scale, it is not surprising that neither the cement 

 of the conglomerate nor the phyllitic interbeds are highly feldspathic. It is 

 clear, on the other hand, that the feldspathic grits and sandstones of the over- 



