E. MINERALOGY AND GEOLOGY. 229* 



many of these valleys. Often the valleys are so choked with 

 the materials thus acciunulated by the action of ice that it 

 is with great difficulty one travels across them. The evi- 

 dences of the glacial epoch are abundant, but they are found 

 only high up on the Uinta Range, and the gravels and bould- 

 ers of the low land are testimony only to the ordinary action 

 of drift agencies. The gravel beds on the low lands are true 

 dviil.FoweWs Bej^ort o/ 1876, p. 171. 



SUB-ALPIXE LAKE BASINS DUE TO GLACIAL EROSION. 



M. Istaldi, in discussing the subject of the glaciers of the 

 Pliocene epoch which have been studied by Desor in Switzer- 

 land, shows, (1) That the existence of the sub-alpine lacustrian 

 basins is intimately connected with that of the glaciers at the 

 epoch of their greatest extension ; that, in fact, the Alpine 

 valleys and the Apennine valleys, and the basins of the lakes 

 of Northern Italy, are all basins of erosion ; that, (2) On the 

 other hand, the thirty valleys which pour their waters into 

 the great basin of the Po open into only six moraine amphi- 

 theatres. (3) Alt sub-alpine lakes are situated within these 

 six amphitheatres, which are the work of the ancient gla- 

 ciers. (4) There is no moraine amphitheatre without lakes. 

 (5) There is no trace of lakes beyond the limits of these am- 

 phitheatres. (6) The breadth and length of the lakes are pro- 

 portional to the dimensions of the moraine ampliitheatre, 

 which is itself in proportion to the dimensions of the zone of 

 sedimentary earth. He concludes, therefore, that if the gla- 

 cier period had not taken place, there would have been no 

 sub-alpine lakes. An examination of the physical conditions 

 of the Pliocene, and the fauna which they contain, shows that 

 the climate was at this epoch warmer and dryer than to- 

 day. Atti della Roy. Acad, della Scienza^ Turin^ 1875, X., 

 490. 



THE DEPOSITION OF FINE SEDIMENTS. 



Dr. T. Sterry Hunt states, in reference to the question of 

 the deposition of fine mud in the Mississippi, that the depos- 

 ited matter requires from ten to fourteen days to subside ; 

 but that if sea-water or salt or sulpliuric acid be added to 

 the turbid water it becomes clear in from twelve to eiohteen 

 liours. Thus is explained the rapid precipitation that occurs 



