CLASSIFICATION OF THE CINNABAR FORMATIONS. 353 



Overlying these Jurassic shaly limestones is a hard ledge that is often a 

 coarsely crystalline limestone, passing into sandstone and even into grits and 

 conglomerates. It is characterized by an abundance of shell fragments, with 

 many specimens of Rhynchonella myrani, Camptonedes, and other Jurassic 

 forms. This horizon is very persistent, occurring wherever the Jurassic has 

 been identified in the mountains of Montana, and forms a very useful datum 

 plane in looking for the coal-bearing strata. 



The conglomerate bed (number 6 of the section) undoubtedly represents 

 the Dakota, but no fossils have been found either in it or in the transitional 

 sandstones. Above this Dakota conglomerate there is a very persistent bed 

 of limestone (number 9 of th^Q section), which is distinguished by great 

 numbers of small gasteropod shells, undoubtedly a fresh-water species, but 

 as yet neither identified nor described. Above the limestone lies a bed of 

 very dense quartzite (number 10), which has yielded no fossils. 



In the shaly sandstones above the quartzite (number 12), specimens of the 

 peculiar Ostrea anomioides have been found, both in this and in other locali- 

 ties. Above these shaly sandstones there is a series of beds which, in this 

 section, are not like typical Fort Benton shales, but are harder and more 

 sandy and include sandstone layers. Number 21 of the section is, ho-w- 

 ever, a crumbly, black, bituminous shale, weathering down to a black earth, 

 and these beds gradually pass into sandy shales, often rather hard, and 

 weathering out as ledges. Where this series (numbers 21 to 26) is exposed 

 in a cliff face, as is the case on the eastern side of the Electric-Cinnabar 

 mountain ridge and in the canon of Gardiner river, inside the national park 

 boundary, they appear as leaden-gray, thinly bedded muddy limestones, with 

 square jointing and numerous harder browu layers which sustain the vertical 

 face of the cliff, the whole exhibiting the usual facies of the marine Creta- 

 ceous series. These beds form the sag south of Cinnabar mountain. 



Above these muddy sandstones and shales there is an abrupt change in the 

 sedimentation to creamy white, quite pure sandstones (numbers 27 to 29). 

 The coal seams of workable thickness all occur in this series of very light- 

 colored, cross-bedded sandstones, which aggregate some 600 feet in thick- 

 ness. Although generally rather soft and sometimes loosely compacted rock 

 easily crumbled between the fingers, these sandstones frequently form con- 

 spicuous bluffs, the underlying beds often weathering into steep and bare 

 slopes capped by mural ledges of this sandrock series. 



These coal-measure sandstones are very generally cross-bedded, while a 

 predominating leafy structure is conspicuous on weathered surfaces, the rock 

 splitting into plates of one-half or even one-quarter of an inch in thick- 

 ness. This structure produces very picturesque forms of weathered ledges. 

 Although the coal-measure series is generally sandy, there are thin belts of 

 shale and clay associated with the many thin seams of coal. 



