GKADATIONS FROM COKTINEK-TAL TO MARINE CONDITIONS IN CENTRAL MONTANA. 17 



TERTIARY (?) SYSTEM. 



LANCE FORMATION. 



Character and thickness. — The section given 

 below is fairly representative of the lower part 

 of the Lance formation, the only part exten- 

 sively represented in this area. This part con- 

 sists mainly of brownish massive to heavy- 

 bedded sandstone and interbedded light-gray to 

 dark-gray shale. Some of the sandstone layers 

 are well consohdated and form protecting caps 

 to the softer underlying beds. Large rusty- 

 broA\Ti logiike concretions are common. Though 

 bro^\^l is the prevailing color, some of these 

 sandstones are dirty white to gray. Ripple 

 marks and cross-bedding are common struc- 

 tural features. 



The interbedded shale is light-gray to lead- 

 gray and contains some carbonaceous members, 

 though these are rare. The shale is uncon- 

 solidated and weathers rapidly where not pro- 

 tected by the more resistant sandstone. Thin 

 layers of impure fossiliferous limestone occur 

 in a very few places. 



Li the northeast corner of the area the beds 

 above described are overlain by a zone con- 

 sisting chiefly of "somber" gray shale with 

 interbedded layers of carbonaceous shale and 

 yellow and bro\vn sandstone. Some of the 

 sandstone layers, which are as much as 20 feet 

 thick, are well consolidated and resemble in 

 every way the sandstones in the lower part of 

 the formation. 



There is no abrupt change between this mem- 

 ber and the lower member described above. 

 They seem to represent continuous deposition 

 and a gTadual change from the conditions 

 favoring the accumulation of sandstone to those 

 favoring the accumulation of shale. This shale 

 member, although not definitely included in the 

 Lebo shale member of the Little Sheep Moun- 

 tain field, seems to be identical in character with 

 it. So far as the writer was able to ascertain, 

 this member pinches out or at least thins 

 markedly farther west; however, more detailed 

 work on the overlying beds and a careful study 

 of the extent and character of the various 

 phases are necessary in order to establish their 

 continuity and determine their relations. 



Section of a part of the Lance forynation exposed in Table 

 Butte, in the SE. i sec. 32, T. 7 N., R. 41 E. 



Feet. 

 Sandstone, massive, cross-bedded, containing clay 



balls 25 



Shale, becoming sandy toward top 80 



Shale, carbonaceous 1 



Shale, yellow to drab 70 



Sandstone, massive, buff; contains bone fragments and 



small scattering pebbles 30 



Shale, buff 20 



Sandstone, massive, brown 10 



"Somber "and yellow clay shale, alternating 60 



296 



Composition. — The sandstones of the Lance, 

 including those interstratified in the dark- 

 shale member described above, as well as 

 those which make up the bulk of the lower 

 distinctly sandy portion of the formation, are 

 arkosic and are sho^vn on microscopic examina- 

 tion to consist of the same material as the 

 sandstones of the Judith River formation, 

 the only difference being the higher degree of 

 oxidation of the material of the Lance. In 

 all the thin sections examined the principal 

 granular minerals present are feldspar, chert, 

 quartz, muscovite, and biotite, named in the 

 order of their abundance ; these are embedded 

 in a matrix of calcium carbonate, which con- 

 stitutes 50 per cent or more of the bulk of the 

 rock. Grains of limestone are also sparingly 

 present in some specimens. The feldspar, 

 including orthoclase, plagioclase, and micro- 

 cline, is commonly the predominant granular 

 min^al. Many of the grains are compara- 

 tively fresh and unaltered and show cleavage 

 or crystal boundaries and beautifully marked 

 striations; others are more or less cloudy and 

 opaque from the development of kaolin. 

 Most of them are angular or subangular, 

 well-rounded grains being exceptional. The 

 chert, which in the hand specimen is gray 

 or black, is invariably present. In the thin 

 section it is clear and transparent and exhibits 

 a cryptocrystaUine texture. It is variable 

 in amount, being in some specimens ahnost 

 as abundant as the feldspar but in others 

 less abundant than the quartz. The grains 

 of chert show a much. more marked tendency 

 toward rounding than those of feldspar. 



