THE MIOCENE OF EUROPE, ASIA, AND NORTH AMERICA 279 



Hatcher ' compares the conditions which led to the formation of these 

 great deposits with those at present prevaiUng along tlie Parana and Para- 

 guay rivers in South America. Citing from Mr. H. H. Smith, he says: 

 "The flood plain of the Paraguay has a width of one hundred and fifty miles, 

 which broadens as we ascend the river, so that the flood plains of the Upper 

 Paraguay, Amazon, and Orinoco rivers are confluent, and that a vast 

 region about the headwaters of these streams presents similar physical 

 conditions. Here," observes Hatcher, "we have a region of equal or 

 greater area than that occupied by Miocene deposits of our western plains, 

 with all the conditions necessary for the deposition and present distribution 

 of the sandstones, clays, and conglomerates, together with the preservation 

 of remains of the faunas characteristic of each." • 



The geology of the Lower, Middle, and Upper Miocene formations in 

 Nebraska, South Dakota, Wyoming, Colorado, Montana, Oregon, and 

 Texas still requires further working out. The section taken by Gidley ^ 

 on the Llano Estacado or Staked Plains of Texas (Fig. 167) illustrates 

 how, through the shifting of streams, old formations begin to be worked 

 over into new. Thus the Upper Oligocene Brule Clays are invaded by 

 Lower Miocene streams of the Arikaree; Middle Miocene formations break 

 down to form materials for Upper Miocene (Fig. 167), and in turn all 

 these older formations may contribute to the subsequent Pliocene and 

 Pleistocene. In general the Miocene overlaps the Oligocene eastward and 

 extends south into Texas and north into Montana. To the north it is 

 much cut up by erosion, to the south and east much buried by later sedi- 

 ment. It is largely composed of altered or eroded volcanic ash materials; 

 in some places pure volcanic ash beds are found. The general direction 

 of the streams was from west to east, or from the mountains out upon the 

 plains. Thus the Miocene becomes finer and thinner as we proceed east- 

 ward, while the coarse materials and sedimentary sands and clays are 

 found closer to the mountains. To a certain extent the evidence points to 

 the same east and west stream channels as exist to-day.^ 



The Mountain Region 



Oligocene and Miocene of Montana} — In Montana the Rocky Mountain 

 chain extends far to the west, and with it the outlying Tertiaries. The 

 ancient flood plain and lacustrine deposits occupy the large valleys of the 



' Hatcher, J. B., Origin of the OHgocene and Miocene Deposits of the Great Plains. 

 Amer. Philos. Soc. Proc, Vol. XLI, 1902, pp. 113-131. 



^ Modified (1908) from section in: Gidley, The Fresh-water Tertiary of Northwestern 

 Texas, American Museum Expedition, 1899-1901. Bull. Amer. Mus. Nat. Hist., Vol. XIX, 

 1903. pp. 617-635. 



' From notes by W. D. Matthew. 



^ Douglass, The Neocene Lake Beds of Western Montana and Descriptions of Some New 

 Vertebrates from the Loup Fork. Univ. Montana, thesis, June, 1899. 



