142 NINTH REPORT. 



PEE-GLACIAL RAINFALL. 



My object in going into some detail regarding the Alma channel is to 

 gather such information as is obtainable concerning the amount of rainfall 

 subsequent to Paleozoic time and antecedent to the glacial epoch Avhen 

 beds of soils and subsoils were in process of formation. That precipitation 

 took place at very early geological times is indicated by the beds of glacial 

 tleposits formed during Cambrian time at the base of the Paleozoic forma- 

 tion and in Permian beds at the close of the Paleozoic. Only recently Mr. 

 A. P. Coleman has described in the March number of the "American Journal 

 of Science " boukler bearing rocks in the Huronian formation of Canada 

 which may represent glacial deposits of very early age. 



Calculating as nearly as possible the amount of rock removed by erosion in 

 the Alma channel, the amount of water which is necessary to remove a cubic 

 foot of rock material, the relation of rainfall to run-off by means of which 

 erosion was accomplished, the relative area in which precipitation and active 

 erosion took place, and the uncertain duration of time during which erosion was 

 acting, we have the factors by means of which the amount of annual rainfall 

 may be uncertainly estimated during the period subsequent to the elevation 

 of Lower Michigan above the sea at about the close of the Paleozoic and pre- 

 vious to the glacial epoch. Without going into all the calculations the re- 

 sult arrived at amounted to about 29 or 30 inches annually for one sc^uare 

 mile for Mesozoic and Cenozoic times. I believe that the present amount of 

 rainfall is given as 32 inches for Lower Michigan. The factors used are: 

 2611 cubic feet of water is required to remove one cubic foot of sediment over 

 a drainage-erosion area of 1,244,000 square miles or less; 16,355,339,000 

 cubic feet of sediment in one sc[uare mile 700 feet deep; the runoff varies 

 to the rainfall as from 33 to 56%, the average of 12 streams being 46%; the 

 period of time is 2,450,000 years, more or less. The most uncertain factors 

 in this calculation are the amount of rock removed by erosion, the relation 

 of rainfall to run-off at that time, the diu'ation of time, and the relation of 

 active erosion to the entire basin of the Mississippi, which is one of the es- 

 sential elements in this calculation. This last subject is worthy of additional 

 investigation on the part of physical geographers and geologists. Wliile 

 these speculations might arouse the envy of the amalgamated nerve of E. H. 

 Harriman and a certain proportion of the AVall Street organization of meta- 

 phorical quadrapeds I trust that it will lead to further investigations and 

 results that will give material for comparison and more accurate informa- 

 tion. 



TILTING OF THE GREAT LAKE BASINS, SHORE LINES, WILLOWY DRAINAGE- 



This subject of tilting of the Great Lake basin was first discussed by G. 

 K. Gilbert in the 18th annual report of the U. S. Geological Survey, the re- 

 sult being that if a line is drawn N. 10° E., 100 miles long, that at the end of 

 100 years the north end of this line would rise .4 of a foot relative to the 

 south end. The nodal line of stability is calculated to pass from Port Huron 

 to Saginaw and thence northwest towards Manistee, the land rising to the 

 north of that line and sinking towards the south. For further information 

 of the details of this interesting subject the reader is referred to the reports 

 on the geology of Huron and Bay counties of the Michigan Geological Sur- 

 vey. 



The south shore of Lake Superior is apparently sinking in the Porcupines. 



It is an obvious corollary from this^ that the river valleys would have 



