NO. 4 CAMBRIAN AND PRE-CAMBRIAN AT HELENA 269 



"Pilgrim limestone!^ — The Pilgrim limestone consists of massive 

 beds of bluish to dark-gray limestones. The lowest bed is a dark- 

 colored crystalline rock, mottled with yellow and dark-gray spots ; its 

 peculiar coloration and massive character are characteristic of the 

 limestone throughout Montana. This bed of mottled limestone is 

 overlain by light-gray to white, non-crystalline limestone, used for 

 making quicklime in the Grizzly Gulch kilns. No fossils have been 

 found in the mottled limestone, but its position and lithologic char- 

 acter correlate it with the ' Mottled ' limestone of the Yellowstone 

 Park folio. This formation occurs on the very summit of Mount 

 Helena, where it forms the uppermost bed of the gentle syncline 

 sweeping down the southeastern side of the mountain. It is also seen 

 in bluffs above the East Side reservoir, and forms a low cliff extend- 

 ing up Oro Fino Gulch for 2 miles above the city, the relief being 

 due to the crumbly nature of the Park shale in which the gulch is 

 being eroded. The mottled beds are 150 feet thick and are overlain 

 by white limestone, which is included in the formation. The total 

 thickness of the formation is 317 feet. 



'' Dry Creek shale.^ — The Dry Creek shale consists of light-colored 

 brownish-yellow, red, and pink shales and calcareous sandstones. 

 The formation is well exposed in few places, but can be recognized by 

 its topographic relief, as it forms sags in the high ridges and ravines 

 on the mountain flanks. No fossils have been found in the shale. 

 It is correlated on the basis of lithology and stratigraphic position 

 with the Dry Creek shale of the Threeforks and Little Belt regions. 

 The thickness is estimated at 40 feet. 



" Yogo limestone.^ — The Yogo limestone consists of light-colored, 

 thin-bedded limestones, wdth crinkly bands and films of jasper, in 

 many places composed of limestone pebbles held in a glauconitic 

 matrix. The formation corresponds to the so-called ' Pebbly ' lime- 

 stone of the Threeforks folio. It has a thickness of 175 to 450 feet. 

 The jaspery, flaggy limestone forms prominent buttress exposures 

 along the east side of Oro Fino Gulch above the city." 



SUMMIT OF THE CAMBRIAN 

 On the map accompanying Bulletin' 527 the line between the Cam- 

 brian and Devonian rocks was supposed to be at the upper Hmit of 

 a well-marked shale deposit. Later work, however, after the map 

 was made, showed that the limestones above this shale also belonged 

 to the Cambrian, and that all of the south and southwest slope of 



Bull. 527, U. S. Geol. Surv., p. gi 



