354 



SCIENCE. 



[N. S. Vol. XVII. No. 426. 



at the Convention would occupy a column or 

 two of Science. 



CYCLES OF PEECIPITATIOJSr IN THE UNITED STATES. 



In the Monthly Weather Review for Octo- 

 ber, Mr. L. H. Murdoch, Section Director of 

 the Weather Bureau at Salt Lake City, con- 

 siders the cycles of precipitation at that station 

 and at other places. He finds for Salt Lake 

 City a dry cycle between 1827 and 1864, dur- 

 ing which the average annual rainfall was 

 about 15 inches; a wet cycle from 1865 to 

 1886, with an average annual precipitation 

 of 18.42 inches, and from 1887 to the present 

 time a dry cycle, the average annual precipi- 

 tation from 1887 to 1901 being 15 inches. 

 From the records for San Francisco, Sacra- 

 mento, Denver, Omaha, St. Louis, Cincinnati 

 and Baltimore it appears that the country 

 west of the Rocky Mountains had its wettest 

 cycle from 1866 to 1887, while the middle 

 Mississippi and Ohio valleys received their 

 heaviest precipitation from 1840 to 1859. The 

 present dry cycle is general from San Fran- 

 cisco to Baltimore. Mr. Murdoch finds no 

 relation between his rainfall curves and Wol- 

 f er's sunspot tables, and concludes ' that there 

 is no known natural law by which we can pre- 

 dict the length of the present dry cycle.' 



The rainfalls for certain stations in the 

 United States, it may be recalled, have lately 

 been studied by Briickner, who finds that they 

 correspond very well with his thirty-five-year 

 climatic period. Mr. Murdoch makes no 

 reference to Briickner's work along these lines. 

 R. DeC. Ward. 



CURRENT NOTES ON PHYSIOGRAPHY. 



ABANDONED CHANNELS OF THE MONONGAHELA. 



The Masontown-Uniontown folio of the 

 Geologic Atlas of the United States by Camp- 

 bell describes a part of the Alleghany plateau 

 in southwestern Pennsylvania. The higher 

 plateau, east of Chestnut-Laurel ridge, is re- 

 ferred with some doubt to a much wasted 

 stage of the uplifted Cretaceous peneplain of 

 the Appalachian province; the lower uplands, 

 further west, represent an Eocene peneplain, 

 now maturely dissected. The chief river is 

 the Monongahela, whose curving valley had 



been already well graded and opened by early 

 glacial times; since then the river has cut a 

 narrow trench 150 feet below its former valley 

 floor. The trench is still so young that only 

 slender discontinuous strips of flood plain are 

 developed along it, on the inner side of 

 curves; while the larger side streams enter 

 the main valley with a strong slope, and still 

 preserve the open flood plains of the earlier 

 cycle in their middle course. But the most 

 peculiar features of the district are the aban- 

 doned channels of the Monongahela at the 

 level of the open valley floor. These are not 

 normally cut-off, round-about channels, like 

 those of the Meuse and Moselle, abandoned 

 by wearing through the necks of the spurs 

 that the river once contoured; for the new 

 courses of the Monongahela are cut through 

 broad, stout spurs for distances of a mile or 

 more. Moreover, the abandoned channels are 

 much clogged with silt, sand and gravel, with 

 some boulders, to depths of 100 feet. Fea- 

 tures of this kind are knovsm in connection 

 with several other north-flowing rivers not 

 far south of the glaciated area, the most noted 

 example being the heavily silted Teay valley, 

 from which the Kanawha has turned north- 

 ward to the Ohio. Campbell suggests that 

 the new courses were taken when the old 

 valleys were locally obstructed at various 

 points by ice dams during the Kansan glacial 

 epoch; each dam is supposed to have gained 

 such strength that it endured for many years, 

 and such height that it surmounted the level 

 of some saddle among the hills on one or the 

 other side of the main valley. Then silts and 

 gravels were deposited in the ponded part of 

 the river, while the new channel was incised 

 in the saddle of overflow. The uplift by 

 which the deepening of the new valleys below 

 the older ones was brought about is dated as 

 post-Kansan. 



The hypothesis of local ice-dams, begun 

 during the spring floods of frozen rivers and 

 strengthened on account of the more severe 

 climate of the early glacial epoch, seems at 

 first reading hazardous frora the number, 

 height and duration of the dams required. 

 The number of examples is, however, more in 

 favor of the hypothesis than against it: if 



