108 DEPARTMENT OF THE INTERIOR 



2 GEORGE V., A. 1912 



Apart from the concretions found in certain layers, the limestones are in 

 many respects similar to the staple phase of the Sheppard formation. The 

 high specific gravity of some specimens, 2-826 to 2-871, shows that they are 

 very high in magnesia or iron and probably approximate ideal, though some- 

 what ferruginous dolomite. In any case all the carbonate bands are rich in 

 magnesia. 



The concretionary structure noted in the thickest dolomitic stratum is a 

 constant feature but is not always typically developed. Though the con- 

 cretionary masses strongly resemble type specimens of Cryptozoon, there seems 

 to be no reason to regard them as of other than inorganic, metamorphic origin. 

 They are spheroids or ellipsoids composed of dolomite in concentric layers 

 separated by thin laminae of cherty silica. The diameter of the bodies varies 

 from a few inches to a foot or more. (Plate 14, B.) Similar, though smaller con- 

 cretions were found in the basal beds of the Sheppard formation m the Clarke 

 range. 



The upper member was estimated to be 1850 feet thick. It is a fairly 

 homogeneous mass of thin-bedded, highly silicious metargillite, interstratified 

 with subordinate, more or less sericitic metasandstone. On a fresh fracture both 

 rock types are generally light gray or greenish gray, the metargillite naturally 

 being of somewhat darker tint. The weathered surface may be gray, brownish 

 gray, rarely red or reddish brown. The member is more ferruginous toward 

 the top. Ripple-marks, rill-marks, sun-cracks, and casts- of salt-crystals up to 

 2 cm. or more in diameter, are all exceedingly common throughout this member. 

 The salt-crystal casts were not found in the lower member. 



Under the microscope these rocks show great similarity to the chief phases 

 of the MacDonald formation. In all the slides, though especially in the more 

 quartzitic types, feldspar is seen to be present. Orthoclase, microperthite, and 

 plagioclase (probably andesine) form a considerable percentage of the clastic 

 grains. A few broken zircons and tourmaline crystals were observed. Sericite, 

 chlorite, and secondary quartz have replaced the original argillaceous matter. 

 The specific gravity of seven type specimens varies from 2-643 to 2-701, with 

 an average of 2-676. The average for the formation as a whole is about 2-680. 



The stratigraphic position, chemical composition, and occasional concre- 

 tionary structure of the lower, dolomitic member are features directly correlat- 

 ing that member with the Sheppard formation of the eastern ranges; the 

 Sheppard thus thins rather rapidly to the westward. The thick upper member 

 of the Gateway carrying abundant salt-crystal casts, is almost certainly of 

 contemporaneous origin with the lower part of the Kintla and, like the Kintla, 

 was doubtless deposited as a continental deposit in an arid climate. 



Phillips Formation. 



The Gateway beds are specially ferruginous toward the top, where they 

 gradually pass into a still more ferruginous mass of sediments. From its 

 occurrence on two summits about two miles north of Phillips creek, this assem- 



