556 STUDIES FOR STUDENTS 



Spokane shales, Helena region, fifteen miles east of Helena. — The Spokane 

 shales occur as massive beds of silicious and arenaceous shales of a deep red 

 color. The arenaceous shaly portions frequently thicken up into thin layers 

 of sandstone. The shales break down on exposure, but they are usually suffi- 

 ciently firm to resist erosion and form strongly marked slopes and cliffs.^ 



The present writer had an opportunity in 1901 of examining the 

 Spokane about 20 miles northwest of Helena and found the shaly 

 layers frequently mud-cracked. 



Grinnell argellite, Lewis and Livingston Ranges. — A mass of red rocks of 

 predominantly shaly argillaceous character is termed the Grinnell argillite from 

 its characteristic occurrence with a thickness of about 1,800 feet in Mount Grin- 

 nell. These beds are generally ripple-marked, exhibit mud-cracks and the 

 irregular surfaces of shallow water deposits. They appear to vary considerably 

 in thickness, the maximum measurement having been obtained in the typical 

 locality, while elsewhere to the north and northwest not more than 1,000 feet 

 were found. It is possible that more detailed stratigraphic study may develop 

 the fact that the Grinnell and Appeknuny argillites are really phases of one 

 great formation, and that the line of distinction between them is one diagonal 

 to the stratification. The physical characters of the rocks closely resemble 

 those of the Chemung and Catskill of New York, and it is desirable initially to 

 recognize the possibility of their having similar interrelations.^ 



Empire shales, Helena region, twenty miles northwest of Helena. — These are 

 greenish-gray massively bedded, banded, silicious shales. 



Helena limestone, Helena region, Helena. — The Helena limestone formation 

 is composed of more or less impure bluish-gray and gray limestone, in thick 

 layers, which weathers to a buff and in many places to a light gray color. Irreg- 

 ular bands of broken oolitic and concretionary limestone occur at various hori- 

 zons. Bands of dark and gray silicious shale and greenish and purplish argil- 

 laceous shale are interbedded in the limestones. These bands are from half 

 an inch to several feet in thickness. There are also beds of thinner bedded 

 limestones, especially toward the top of the formation. ^ 



Siyeh limestone, Lewis and Livingston Ranges. — Next above the Grinnell 

 argillite is a conspicuous formation, the Siyeh limestone, which rests upon the 

 red shales with a sharp plane of distinction, but apparently conformably. The 

 Siyeh is in general an exceedingly massive limestone, heavily bedded in courses 



two to six feet thick like masonry Occasionally it assumes slabby 



forms and contains argillaceous layers. It is dark blue or grayish, weathering 

 buff, and is so jointed as to develop large rectangular blocks and cliffs of extraor- 

 dinary height and steepness. Its thickness, as determined in the nearly vertical 

 cliff of mount Siyeh, is about 4,000 feet."^ 



1 Bulletin Geological Society of America, Vol. X, p. 207. 



2 Ibid., Vol. XIII, p. 322. 



3 Ibid., Vol. X, p. 207. 4 Ibid., Vol. XIII, p. 323. 



