Geology and Mineralogy. 171 



the folio. The area therefore, lies immediately north of the Yel- 

 lowstone National Park. The southeastern portion comprises an 

 elevated block of mountains called the Snowies, chiefly of 

 Archaean formation but cut by intrusions of igneous rocks and 

 covered in part by enomerous extensions of lavas and tuffs. 

 Around the western and northern slopes of this great mass winds 

 the Yellowstone river in an open valley. The Paleozoic forma- 

 tions lie tilted up against its flanks, while the Mesozoic are partly 

 upturned and partly occupy in more horizontal position the open 

 valley of the Yellowstone. The western edge is defined by the 

 slopes of the Gallatin Range, chiefly composed of igneous extru- 

 sive masses, and the Bridger Range of Archaean and Paleozoic 

 upturned rocks. To the north the line passes through the south- 

 ern flanks of the Crazy Mt. massive, Cretaceous ? strata injected 

 with igneous masses of which only the singularly vast and intri- 

 cate system of radiating dikes shows on the map. 



Thus there is great variety both in topographic form and in 

 the range of geological formations, affording an excellent oppor- 

 tunity for the display of the varied resources of the survey in 

 geological work and map-making. 



The folio contains first two pages of text explanatory of the 

 geography, topography and geology both scientific and economic. 

 Then follows a topographic map with the elevations given by 

 contours and on a scale of four miles to the inch. These maps pro- 

 duced by the survey, often in conjunction with the States, have 

 become tolerably familiar to the public of late years. When the 

 amount of time and money available for their production is taken 

 into consideration they are wonderfully accurate, while the 

 mechanical execution is extremely perfect and beautiful. The 

 triangulation and topography of the Livingston sheet are by 

 Messrs. Renshawe, Douglas, Tweedy and Leffingwell, under 

 the supervision of Mr. Gannett, chief topographer and Mr. 

 Thompson, chief geographer in charge. 



The geological maps in colors that follow are three in number, 

 the first giving the areal geology on the topographic base, the 

 second devoted to economic geology is similar but with the coal- 

 bearing strata strongly accentuated, while the third on a flat base 

 gives only the main geologic areas and is cut by sections giving 

 the geological structure in detail. 



These are followed by a plate giving the columnar section of 

 the sedimentary formations accompanied by full notes and details. 



The geological portion of the work has been under the charge 

 of Arnold Hague, the geology in the field having been done by 

 Messrs. J. P. Iddings and W. H. Weed. The names of these 

 specialists are sufficient to indicate the clear and excellent manner 

 in which the survey has been carried out, and the maps them- 

 selves give the best evidence of the careful and painstaking work 

 that has been put upon them both in the field and in the office. 



The pleasing tones and patterns and the artistic combinations 

 of colors of these maps deserve a separate word of commenda- 



