A Discussion of the Geology and an Isopach Map of the Pennsylvanian System in Wyoming and Adjacent Areas

Pennsylvanian strata in Wyoming and adjacent areas have been the subject of much work and discussion. Most of the work has been due to the economic importance of the system as an oil producer in this region. Oil production from strata of Pennsylvanian age is rather recent history; and therefore, much of the available information is local­ized and incomplete.

The Chromite Deposits of Stillwater and Sweetgrass Counties, Montana

In writing this report, two objects were kept in mind, (1) to explain, if possible, the origin of the chromite deposits found in Sweetgrass and Stillwater Counties, and (2) to bring up to date all information on these deposits which had thus far been available. The work done consisted of study of the rocks and ores of the area under the microscope, both as thin sections and as polished sections, practically all of which was done at the Montana State School of Mines, during the school year of 1928 - 1929. The rock specimens and much information as to their locations and probable compositions were obtained from Mr. P. F. Minister, of the East Butte Copper Company. United States Geological Survey Bulletin 725-A, Deposits of Chromite in California, Oregon, Washington, and Montana, and the unpublished report on the Chromite deposits of the Boulder River, prepared by Prof. C. H. Clapp of the University of Montana, were frequently referred to and considerable material was drawn from them. The map of the Boulder River area is from Clapp's report.

The Aplitic Body Bordering Roosevelt Drive South of Butte, Montana

The area studied consists of a large body of aplite situated 12 miles south of Butte. It extends eastward almost to the Butte-Pipestone road, southward to Lime Kiln Hill and westward a distance of 3000 feet. The petrographic work was supplemented by a map of the area. Special attention was given to those frequent "Limonite Specks" found in the aplite.

The Calcite Veins of the Livingston Formation

An interesting group of calcite veins occur near Livingston Montana in a zone about eight miles wide and forty miles long in the edge of the Plains region of Montana in front of the Main ranges. The zone extends from the Boulder River south of Big Timber, through Springdale and Hunters Hot Springs, to Potter's Basin just north of Wilsall in Park County. The group of veins is particularly interesting because they cut relatively flat lying strata, suggest a structural relation­ship to one another, and they are nearly pure calcite. The present investigation was to determine the position of the calcite veins by plotting them on a base map of the district. In addition, it was planned to determine the mineralogy end origin of the veins and their structural and stratigraphical relationship to the rocks of the Livingston formation which they cut.