1897.] NATURAL SCIENCES OF PHILADELPHIA. 433 



was before. The combined effect of these two movements would be 

 to drown the upper course of these streams and favor the forma- 

 tion of lakes." 



The hypothesis first referred to, for which I find that I am re- 

 sponsible, is built upon the admitted structure of the river beds in 

 their divisions described above, and is in effect that at some time 

 posterior to the deposition of the rocks of the middle course, a 

 profound dislocation occurred along the Ourals involving an up- 

 throw of the entire eastern half along a line of fault parallel to and 

 not far from the axis of the chain, which brought the underlying 

 Archean rocks to the level of the Devonic of the western slope. The 

 entire series of rocks forming the eastern slope as far as the present 

 westernmost occurrence of the rocky gorges of the middle slopes were 

 affected by this movement. The high angle and great precipitation 

 cut channels for the rapid streams directly eastward, and ploughed 

 out the canons in the Tertiary rocks of the Siberian plains. A 

 period of erosion ensued during which the elevated eastern half of 

 the Oural was greatly reduced in height. Following this was a 

 down-throw of less extent than the original elevation but of sufficient 

 extent to reduce the rapidity of flow of the rivers near their sources 

 and on their upper courses, and to transform these latter more or 

 less into morasses and swamps. In time the sunken river beds of 

 the upper courses werefilled by sediments, while the rocky gorges of 

 the middle courses remained as before the channels of streams no 

 longer possessing sufficient rapidity to have cut them. It seems 

 reasonable to suppose that if there had been such movements, they 

 might have produced all the dissimilarity now observable between 

 the two slopes of the Oural, even if the structure of the two sides had 

 been originally similar. The rivers of the east slope prior to the 

 first movement probably originated in the longitudinal valleys 

 of the harder crystalline and quartzite rocks of the east side. 

 Their first courses very likely were rapid and tumultuous and 

 more or less parallel to the axis of the chain, as is now the case 

 with those of the west side, for considerable distances or until 

 favorable places were found for them to break through in lines 

 perpendicular to the axis of the range, when like those of the 

 western slope they may have excavated their beds, first through 

 the older paleozoic rocks and further east through the Tertiary, 

 and finally have reached the level steppes far to the east. The 

 first effect of the elevation would be naturally to produce direct east 



