SOME PETROGRAPHIC DETAILS 463 



summit of the southern laccolith, then called Mount Morris. We have 

 collected at 110 of the map of East Butte a breccia of porphyry which 

 is richly mineralized with cubical iron pj^ites. It came from the Brown- 

 Eyed Queen claim and obviously along a line of movement and crush. 

 Eeports of other sulphides in West Butte are current. 



The most important object of mining today is a seam of coal at the 

 McDermott mine, in the Virgelle or Eagle sandstone, at the southeast 

 corner of West Butte. The seam is 2 feet thick and has been changed 

 by the intrusives into what is locally described as a high-grade semi- 

 anthracite. It is mined in a modest way for local consumption. Of 

 the explorations for oil and gas in the strata surrounding the Hills men- 

 tion will be made in the following discussion of the sedimentary geology 

 by the junior writer. 



Sedimentary Rocks" 



stratigraphy 



Erosion of the Sweet Grass Hills uplift has exposed formations which 

 are buried under later deposits for scores of miles in every direction. 

 (See figure 7.) Thus the stratigraphic sections which can be measured 

 here form a very valuable connecting link in the knowledge of these for- 

 mations. In this lies their chief interest and importance. 



MI881SSIPPIAN 



Tlie section. — The oldest visible member of the sedimentary series is 

 tlie Madison limestone, of Mississippian age, and the youngest is the 

 Judith River meml^er of the Montana group. The section included be- 

 tween these limits comprises the most interesting and economically the 

 most important portion of the entire stratigraphic column in Montana. 

 Al)ove the Madison limestone, elsewhere in the State, are a series of rocks 

 which have yielded petroleum at several horizons and which are at present 

 being actively explored by the drill at scores of places, including the 

 Sweet Grass Hills themselves. Any light that the section here can cast 

 upon the extent, character, and thickness of these formations is there- 

 fore of immediate value. 



'" The subsciiiient pages are by Pan] Billingsley. 



