268 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 64 



Wolsey shale. — The Wolsey shale, Meagher Hmestone, Park shale, 

 Pilgrim limestone. Dry Creek shale, and Yogo limestone are all in- 

 cluded under -€ls on the map. " The Wolsey shale ^ consists of 

 micaceous and calcareous gray to greenish shales, which contain 

 small oval and flat concretions of limestone, grading in places into 

 thin and very irregular plates of limestone. Trilobite and shell re- 

 mains of Cambrian types occur abundantly along the contact between 

 these shales and limestones. The rocks are in few places well ex- 

 posed, owing to their soft and crumbly nature, but their position is 

 recognizable by the ravines cut in them or, on the mountain slopes, 

 by their forming a more gentle angle between the limestone bluffs 

 above and the quartzite ridges below. They have a thickness of about 

 420 feet" (in the Helena district). 



"Meagher limestone.^— The Meagher limestone is composed of 

 light-gray to bluish limestones, which are shaly near the base, but 

 grade into alternating beds of massive, dark-colored and flaggy, white 

 limestones, and these into thinly-bedded, dark-purple to blue, fossil- 

 if erous limestones, forming the top of the series. In other regions the 

 rocks are pebbly, but this character is not conspicuous in the Helena 

 district. The rocks have an estimated thickness of 400 feet. They 

 form the characteristic bluffs on the north face of Mount Helena, 

 extending from the gentle slopes formed by the Wolsey shale upward 

 almost to the very summit of the mountain. The rocks are also seen 

 in the bold cliffs below the east side reservoir. Fossil remains occur, 

 but no collections were made. 



"^ Park shale.' — The Park shale consists of earthy and micaceous 

 dark-gray to green or purple shales. The rocks are not well induratefl 

 and crumble readily, so that very few good exposures are seen. A 

 partial section is exposed in the quarry near the upper part of the 

 city of Helena, and shows the formation to contain lavender or 

 pinkish beds, grading through green shales to a grayish earthy shale 

 carrying an abundance of small fossil shells, identified as Oholella 

 [Obolella (Westonia) ella].^ The upper portion contains limestone 

 lenses in a jaspery shale, which grades downward into a dense cherty 

 rock resembling hornstone. This shale has an estimated thickness of 

 150 feet. It forms the flat bench on the summit of Mount Helena, 

 between the apex and the northern cliff's, and covers the ridge fol- 

 lowed bv the trail. 



* Bull. 527, U. S. Geo!. Surv., p. 90. 

 - Idem, pp. 90-91. 



^The reference now is to Obolus (Wesfo}iia) ella. (Monogr. 51, U. S. 

 Geol. Surv.. IQ12, p. 455.) 



