J. S. Newberry— Flora of the Great Falls Coal Field. 191 



Art. XXL— The Flora of the Great Falls Coal Field, 

 Montana ; by J. S. Newberry. (With Plate XIV.) 



In the School of Mines Quarterly for 1887, I published a 

 brief description of the coal basin which underlies the country 

 about the Great Falls of the Missouri in Montana, and 1 am 

 now able to add some facts of more than usual geological in- 

 terest to those before known in regard to this coal field. 



The Great Falls coal basin lies on the north slope of the 

 Belt and Highwood Mountains ; the strata all dipping toward 

 the north. These mountains are subordinate folds of the 

 Rocky Mountain system and are each composed of a granitic 

 Archaean nucleus, locally overlain by a great thickness of 

 Cambrian rocks which are best seen about Sulphur Springs. 

 This formation must be at least 10,000 feet in thickness, and it 

 underlies the surface from a point fifteen miles north of Sul- 

 phur Springs to near Townsend on the south. Splendid ex- 

 posures of the same group are seen in Prickly Pear Canon 

 on the road from Helena to Great Falls. They consist of 

 numerous alternations of thin bands of fine grained sandstone 

 and argillaceous shale, generally metamorphosed into quartzite 

 and slate. Near Sulphur Springs is an outcrop of limestone 

 converted into marble. The prevailing color of the rocks of 

 this group is gray at the surface, darker below. No distinct 

 fossils were found in the slate though particles of carbonaceous 

 matter abound everywhere. A shaly sandstone which appar- 

 ently overlies all the series described is largely made up of 

 Primordial trilobites. 



On the summit and the north slope of the Belt mountains 

 the Archaean granite nucleus is overlain by Potsdam sandstone 

 full of Scolithus and casts of sea-weeds. There are here nu- 

 merous large dykes of rhyolite which cut the granite and sand- 

 stone. 



Succeeding the Potsdam sandstone is a great mass of Paleo- 

 zoic limestone, sometimes blue, but mainly of a cream color, 

 which has been cut by the streams draining northward into 

 most picturesque cations and valleys of which the sides are set 

 with buttes imitating castles, fortresses, churches, etc., combin- 

 ing to form scenery equally attractive to the tourist and geolo- 

 gist. In the limestones are found both Silurian and Carbonif- 

 erous fossils. North of the mountains the limestones are 

 unconformably overlain by a series of sandstones, shales and 

 fresh water limestones which include one large and several 

 smaller seams of coal. These dip toward the north and are 

 soon covered with a great and continuous sheet of glacial drift 



