18 



SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1919. 



The quartz occurs in subangular to rounded 

 grains. It is variable in amount, being almost 

 wholly absent in one specimen and in others 

 constituting 25 per cent or more of the granular 

 material. Muscovite is present in notable 

 amount and is easily recognized in the hand 

 specimens; biotite is also sparingly pl-esent. 

 The calcite, which constitutes the matrix of 

 the rock, shows a finely crystalline texture. 

 It is commonly more or less stained and 

 obscured by limonitic material, which in 

 most of the specimens also occurs to a greater 

 or less degree in distinct grains. Like the 

 sandstones of the Judith River, those of the 

 Lance are medium grained, and their grains 

 are of fairly uniform size, showing a marked 

 degree of assortment. 



Age and correlation. — Paleontologists are 

 not agreed as to the age of the Lance forma- 

 tion, the invertebrate and vertebrate fossils 

 being regarded as related to Cretaceous tyj)es, 

 whereas the plants are regarded as of Fort 

 Union age. Under these circumstances the 

 Geological Survey has only provisionally as- 

 signed the Lance to the Tertiary system. 



The formation is identified in this area 

 and its relations are established by (1) 

 the finding of Triceratops remains near the 

 base of the formation at Castle Butte, north- 

 west of Forsyth, and the occurrence in it 

 of many fragments of dinosaur bones in other 

 parts of the area; (2) its stratigraphic position 

 between the Bearpaw shale and the Fort 

 Union formation; and (3) its continuity with 

 beds that undoubtedly belong to the Lance 

 in other parts of Montana. 



Distribution and surface features. — The 

 Lance formation occurs at the southern, 

 eastern, and northern margins of this area 

 and extends beyond the area in each of these 

 directions. In general it occupies a zone from 

 1 mile to several miles broad, its width depend- 

 ing on the attitude of the beds and the stage 

 of erosion. 



The outcrop of the formation has a marked 

 effect on land forms. Its lower part produces 

 a prominent ridge which rises abruptly, in 

 some places almost precipitously, from the 

 lower areas occupied by the Bearpaw. From 

 the margin it extends as a dissected upland 

 studded with buttes, pinnacles, and other 

 features characteristic of the weathering of 

 moderately consolidated beds in a semiarid 

 region. 



STRUCTURE. 



In its broad general relations the structure 

 in this part of Montana is anticlinal and rep- 

 resents the southeastward extension of the 

 Big Snowy anticline. More locally the east- 

 ern part of the area occupies the eastern and 

 northern flanks of the Porcupine dome, aroimd 

 which the beds have a quaquaversal dip and 

 are therefore in this area inclined southward, 

 eastward, and northward. As shown on the 

 accompanying map, the dips in this part of 

 the area range from 1° or 2° to 40°. This 

 dome is separated by a broad, shallow trans- 

 verse syncline from a pronounced anticline in 

 the vicinity of Musselshell River. On the 

 west side of the syncline the strata dip L5°- 

 25° E.; on the east side the westward dip is 

 not more than 2° or 3°. West of this cross 

 syncline the Big Sno\\'y anticline is the domi- 

 nant structm"al feature. Around the pomt of 

 this anticline the Judith River formation dips 

 from 5° on the south limb to 45° on the north. 

 These steep dips do not continue for any con- 

 siderable horizontal distance in the direction 

 of the dip, as the overlying Lance rarely shows 

 dips of more than 10° along its border and 

 within half a mile lies essentially flat. It is a 

 noteworthy fact throughout the area that 

 the steeply dipping beds become approximately 

 horizontal within short distances. Tlie fea- 

 tures above described are sho^vn graphically by 

 the structure sections along the lines ABC and 

 EF on Plate IV. 



Besides the main structural features out- 

 lined above there are local irregularities, such 

 as the anticlines in T. 14 N., Rs. 32 and 33, 

 37, and 38 E. 



SOURCE OF THE SAND. 



The feature that most impresses one who 

 studies the sandstones of the Lance and Judith 

 River, and, so far as can be judged by a micro- 

 scopic examination, this applies also to the 

 Eagle sandstone, is their identity in mineral 

 composition and the relative abundance of 

 the predominant minerals. The abundance of 

 feldspar, the invariable occurrence of black 

 chert, commonly in considerable amount, to- 

 gether with quartz and mica, and the almost 

 complete absence of other granular constitu- 

 ents strongly suggest derivation either from a 

 common source or at least from rock of 

 similar composition, and the uniformity in 



