proceedings: geological society 109 



productive territory. In structure the rocks have the form of an arch 

 about 50 feet high on a general eastward dip of about 20 feet to the mile. 

 The oil is found in a soft, porous sandstone in the uppermost part of the 

 Mississippian series, a little over 1000 feet below the surface. There 

 are many layers of barren sandstone above and below, which, like the 

 producing sand, are arched; but they differ from it in that they extend 

 out beyond the borders of the field, whereas the producing sand pinches 

 out on three sides of the pool. On the fourth side this sand is full of 

 salt water. All of the wells yield gas, oil, and water, the initial pro- 

 duction of oil ranging up to 2000 barrels a day. There are now over a 

 hundred wells and the total production is about 6000 barrels a day. The 

 oil has a gravity of 33 to 37 degrees Baume. Most of the wells are shot 

 with about forty quarts of nitro-glycerine. 



An old erosion surface in eastern Utah, its age and value in time deter- 

 minations: Joseph B. Umpleby. During reconnaissance studies in 

 1910 and 1911 an old erosion surface was recognized in eastern Idaho. It 

 was seen to be older than one group of mineral deposits but to truncate 

 the other. Thus its age if definitely established would place limits on 

 two great periods of mineralization. The aim of this paper is (1) to 

 show that an old erosion surface extends over much of Idaho and into 

 adjacent regions; (2) to outline the evidence for the conclusion that it 

 is of Eocene age; (3) to suggest that the elevation of the highlands which 

 were later planed down was accompanied by great granitic intrusions, 

 and (4) to point out the relations of the Eocene surface to the ore deposits. 



1. A correlation is suggested between the erosion surface in eastern 

 Idaho and the plateau-like surfaces that have been recognized in west 

 central Idaho and the Clearwater Mountains by Lindgren, in the Coeur 

 d'Alene range of Idaho and the Cabinet and Purcell ranges of western 

 Montana by Calkins, in the Galton and possibly also the Livingston and 

 Lewis ranges farther east by Willis, in the Interior Plateau of British 

 Columbia, where Dawson describes an Eocene peneplain, and in the 

 Republic district of Washington by the speaker. 



2. Early Miocene sediments, laid down in great erosion valleys de- 

 veloped after the last important elevation of the old erosion surface, indi- 

 cate its pre-Oligocene age, allowing Oligocene time for the development of 

 the valleys. On the other hand the surface cuts all the older formations, 

 including the Idaho granite which is post-Triassic. Furthermore, the 

 Eocene sediments of the Northwest surround the plateau region and 

 were evidently derived from it. They could not have come from this 

 region after its last elevation for two reasons: (a) It is doubtful if 

 there has been sufficient dissection of the plateau to supply the volume 

 of sediments represented, and (b) all the more important later valleys of 

 the region drain westward. The conclusion, therefore, is that the Eocene 

 sediments were derived from the plateau region during that cycle of 

 erosion which resulted in gentle topographic forms, and hence that the 

 old erosion surface is of Eocene age. 



3. The plateau region as before defined is characterized by numerous 

 granitic batholiths which are of about the same age and comprise pos- 



