146 DEPARTMENT OF TEE INTERIOR 



2 GEORGE V., A. 1912 



symmetry a maximum extinction of 20°; it appears thus, to be an acid labra- 

 dorite. The phenocrysts of the porphyritic phases, though singularly hard to 

 diagnose, seem to be of nearly the same species of feldspar. In not a single slide 

 was there found the slightest trace of other phenocrysts. Even pseudomorphs 

 of such possible original phenocrysts as pyroxene or amphibole entirely fail. 

 Judging from the nature of the secondary minerals, the original lava was in all 

 probability a rather basic andesite or andesitic basalt. 



Some of the fine-grained, non-amygdaloidal greenstone may, at certain 

 points in the field section, belong to dikes or sheets of the lava cutting slightly 

 older flows. Largely on account of the profound metamorphism it has proved 

 as yet impracticable to distinguish such possible intrusives in the field. They 

 can, however, in any case, form but a small part of the whole mass. 



The microscopic character of the long list of secondary minerals shows 

 thorough banality and needs no special description. 



The specific gravity of eleven type specimens ranged from 2-791 to 3-096,. 

 with an average of 2-919, which cannot be far from the average for all the 

 greenstone. 



About 2,000 feet below the top of the formation the greenstone is inter- 

 rupted by a forty-foot interbed of compact, somewhat sheared, gray limestone 

 weathering light yellowish or buff. Under the microscope the rock is seen 

 to be a remarkably homogeneous granular aggregate of carbonate grains 

 without other visible impurity than a little granular quartz occupying narrow, 

 microscopic veinlets, cutting the rock proper. The carbonate grains are anhed- 

 ral, roundish, and of nearly uniform size, averaging 0-015 mm. in diameter. 

 The rock effervesces very slightly with cold dilute acid. The specific gravity 

 is 2 -'353, indicating a nearly pure dolomite. The purity of this carbonate mass, 

 coupled with its fineness and uniformity of grain, strongly suggests a chemical 

 origin for the rock. It should be noted that the average size of the carbonate 

 grains is very similar to the average size of the grain in the Altyn, Siyeh. 

 Sheppard, and other magnesian formations of the eastern series. 



The 200-foot breccia-conglomerate occurring near the top of this formation 

 is of special value as a horizon-marker. Because of its high angle of dip and 

 because of its power of resistance to the processes of general erosion, the con- 

 glomerate often projects in strong peaks or ridges above the surrounding green- 

 stone. Fine exposures were found on the summits north of Monk creek and on 

 the long northern slope of Summit creek valley. (Plate 72, B and C.) From 

 the Boundary line to Summit creek this conspicuous rock-bed is always practically 

 vertical and runs in a remarkably straight line, bearing a few degrees east of 

 north. Throughout that stretch there seems to be no possibility of any impor- 

 tant amount of dip-faulting in the Summit series as a whole. The persistence 

 of this clastic bed, both in strike and dip, and its steady parallelism to the 

 boundaries of the other nearly vertical members of the Summit series out- 

 cropping in this area, testify to the conformity of the whole Irene volcanic 

 formation with the Irene conglomerate, and with the Monk and younger forma- 

 tions. Had it not been for the discovery of this band of conglomerate, the 



