GEOLOGICAL IMPORTANCE OF SEDIMENATITON 557 



Sheppard quartzite, Lewis and Livingston Ranges. — A distinctly sandy phase 

 of deposition succeeding the extrusive rhyolitic eruption capping the Siyeh 

 limestone has resulted in a quartzite which is very roughly estimated to have 

 a thickness of -700 feet.' 



The present writer has observed a basal quartzite to the Marsh 

 shales in a similar stratigraphic relation upon Greenhorn Mountain, 

 sixteen miles northwest of Helena. But the occurrence of the quartz- 

 ite was lenslike and not persistent for many miles. 



Marsh shales, Helena region. — At Helena there is a thickness of about 250 

 feet of shales and thin-bedded sandstones of the Belt Terrane above the Helena 

 limestone and beneath the Cambrian sandstones. The same bed, on the north 

 side of Mount Helena, is reduced to 75 feet in thickness, but to the northwest 

 the formation increases in thickness to 300 feet or more.^ 



Kintla argillite, Lewis and Livingston Ranges. — Argillite and quartzite, 

 thin-bedded, maroon red, ripple -marked, and sun-cracked, containing casts of 

 salt crystals; also occasional beds of white quartzite and some calcareous; thick- 

 ness 800 feet; no upper limit seen.^ 



The Kintla formation closely resembles the Grinnell, and represents a recur- 

 rence of conditions favorable to deposition of extremely muddy, ferruginous 

 sediment. The presence of casts of salt crystals is apparently significant of 

 aridity, as the red character is of subaerial oxidation. The formation has an 

 observed thickness of 800 feet, but no overlying rocks were found. Its total 

 thickness is not known, and the series remains incomplete.''^ 



Discussion. — The very similar general nature of these formations 

 at a distance of 150 miles from each other indicates similar conditions 

 of accumulation over wide areas, though it is possible of course 

 that the stratigraphic cycle was not strictly contemporaneous in the 

 two regions. The volume of material which must have been eroded 

 to supply these sediments was far greater than the volume of the 

 sediments, since the one kind of sediments of any epoch, occurring 

 at both localities, represents but a portion and, in the case of the 

 limestones and quartzites, but a small portion, of the rock masses 

 whose erosion supplied the material. 



Taking Clarke 's figures^ of the average amounts of the oxides and 

 common minerals in the "primitive crust of the earth" it is seen 

 that an approximately pure dolomite which should contain all of 



1 Ihid., p. 324. 3 Ibid., p. 316. 



2 Ibid., Vol. X, p. 207. 4 Ibid., Vol. XIII, p. 324. 



5 " Analysis of Rocks, Laboratory of the U. S. Geological Survey," Bulletin 

 U. S. Geological Survey No. 168 (1899), pp. 14, 16. 



