408 ABSTRACTS 



the Algonkian beds were being deposited to the extent of from 6000 

 to 12,000 feet, there was a gradual subsidence of the whole region, and 

 shallow seas for the most part prevailed. During the Palaeozoic age 

 there were many minor oscillations of the surface, which were more 

 frequent during Cambrian time than during the deposition of the 

 Devonian and Carboniferous limestones. Toward the close of the 

 Cretaceous period a general elevation began,, which was accelerated 

 after the deposition of the Laramie formation. The formation of the 

 mountain ranges, together with the subsequent erosion, resulted in 

 many valleys, which eventually were occupied by fresh-water lakes. 

 These lakes attained their greatest extent in the Neocene period, last- 

 ing in all probability until the Pleistocene period was well advanced, 

 and during their earlier stages immense bodies of wind-carried volcanic 

 dust were deposited in their waters, and are now seen as beds of pure 

 white. At the same time the dust fell upon the surrounding country, 

 from which it was afterward washed into the lakes, forming an upper 

 series of yellowish and rusty colored beds. These dust showers 

 destroyed both animal and vegetable life, and the remains carried into 

 the lakes were buried in their deposits, where they are now found as 

 fossil bones and opalized and silicified wood. 



Under the "Description of Rock Formations" are outlined all the 

 formations from the Archean gneisses up through the Algonkian, 

 Cambrian, Devonian, Carboniferous, Juratrias, Cretaceous, Eocene, 

 Neocene, and Pleistocene. The rocks of more than half of the area 

 are of sedimentary origin, while the crystalline rocks occupy approxi- 

 mately 1000 square miles, the remaining third of the area being cov- 

 ered with igneous materials. Prominent among the latter are the 

 andesitic breccias which form the main part of the Gallatin range, the 

 great porphyritic laccolite occupying the center of the Madison range, 

 and the basaltic plateau which lies west of the Madison valley. 



Under the heading "Structural Geology," after a general consid- 

 eration, the vertical and horizontal movements are discussed, and the 

 development of the lake basins is described. The arrangement of the 

 rock-mass is complex, the structure being complicated by laccolites, 

 dikes, and surface flows of igneous material. Unconformities exist, 

 showing that areas previously raised to land surfaces and worn down 

 have subsided, have been crossed by an advancing shore, and later 

 have passed beneath the sea. The great series of conformable strata is 

 closely folded, and has been pushed up in arches, many of which have been 



