ABSTRACTS 409 



overturned from the effect of horizontal thrusts. The simple as well 

 as the overturned synclines are marked by areas of Laramie Cretaceous 

 beds, which, at the time of the folding, were the latest and highest of 

 the formations. The arches between the troughs having been broken 

 and exposed by the elevation, excessive erosion has worn them down 

 to the older rocks, exposing the Archaean, which usually forms the 

 axes of the uplifts. Unlike the Appalachian folds, which are strikingly 

 parallel and continuous, these folds lie in various directions, due to 

 several independent centers of uplift. Three great faults cross the 

 Gallatin range, two of them extending across the Madison range to the 

 extreme western part of the area. Following or accompanying the 

 folding of the Cretaceous and pre-Cretaceous strata the detritus result- 

 ing from the greatly facilitated erosion, together with volcanic material 

 erupted during this epoch, was deposited unconformably on the eroded 

 upturned edges of the earlier-formed strata. 



The lake basins are now the floors of extensive valleys separating 

 the detached mountain ranges, which rise about 6000 feet above their 

 bases. As the lake deposits are at least 2000 feet in thickness, the 

 difference of elevation between the bottoms of the lake basins and the 

 summits of the peaks must be at least 8000 feet. The region was a 

 mo'untainous one before the development of the lakes, but in the evo- 

 lution of the existing relief, movements and erosion have both oper- 

 ated to accent the topographic differences. 



The principal economic resources of this region are gold, silver, 

 iron ore, copper, limestone, and coal. The occurrence of coal in 

 Devonian rocks on the north side of the Jefferson canyon is of geologic 

 interest, although not of any great economic importance. The fine 

 pumiceous volcanic dust found in the old lake basins has been utilized 

 to a very limited extent as a polishing material. Brick clays occur, 

 and are used to a small extent in a few localities, especially near Boze- 

 man. In addition to the economic resources just referred to, the sheet 

 of economic geology has indicated upon it the localities of building 

 stone and mineral springs. 



Geologic Atlas of the United States. Folio 2g, Nevada City, special folio, 

 California, i8q6. 



This folio, by Waldemar Lindgren, consists of seven pages of text, 

 three special topographic maps (scale i : 14,400), the Grass Valley, 



