266 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 64 



and westward of Mount Helena for several miles. Near the railroad 

 about 2 'miles (3.2 km.) west of East Helena they consist of pale, 

 greenish-gray slates with characteristic purple spots. 



Helena limestone. — (Ah on map.) "The Helena limestone is a 

 formation composed predominantly of impure bluish-gray or gray 

 non-crystalline limestone. The limestones occur in beds i foot to 6 

 feet thick and contain thin, interbedded bands of gray siliceous shale, 

 more rarely of green to purple clay shales. The limestones are ordi- 

 narily dark blue on fresh fracture, but show a characteristic bufT- 

 colored, velvety appearing surface on weathering. The upper beds 

 have a rough surface, with a pale or blue-gray color, and resemble 

 Cambrian rocks. These beds alternate with shale and form the 

 ridges on the northwest foot slopes of Mount Helena. 



" The formation has no distinctive physiographic expression within 

 the district, but its relatively massive bedding gives the limestone 

 prominence on the slopes about the city. 



" The name was given the formation by Walcott from its typical 

 occurrence about the city of Helena. 



" The formation is barren of fossils, though the oolitic character 

 and the local presence of carbonaceous markings has led to the belief 

 that they will ultimately be found. The estimated thickness of 2,400 

 feet in this vicinity is based on rough measurements, as it is im- 

 possible to find a satisfactory exposure of the entire formation for 

 exact measurement. The formation covers a large part of the dis- 

 trict, especially about the borders of the Prickly Pear Valley. So far 

 as known the rocks are conformable to the formations above and 

 below, and grade into them by intercalations of shale. The upper and 

 lower limits are therefore not sharply definable. About a mile east 

 of Helena the Marsh shale is wanting and the Cambrian quartzite 

 rests directly on the eroded surface of the Helena limestone. Good 

 outcrops are seen near the high school within the city limits. 



" On the north face of Mount Helena the Cambrian quartzite rests 

 on dark-blue and dense limestones, weathering bufif, and these rocks 

 in turn rest on pink and buff-colored shales, which appear red in 

 most exposures and are included as part of the Helena limestone. 

 These reddish shales contain numerous massive beds of white cherty 

 limestone, forming reefs running obliquely across the slopes and 

 extending downward within a few yards of the streets of Kenwood. 

 The beds of limestone are 6 to 10 feet thick and in weathering and 

 character resemble those of Paleozoic rocks. The same pink and 

 buff-colored shales, with interbedded limestones, are seen in the 



