Geology and Mineralogy . 227 



Among other conclusions Dr. Gilbert finds reason to believe that 

 a portion of the survey of 1842 was in error. The rate of reces- 

 sion of the American Falls is also studied and is considered an 

 important part of the problem, of the time required for cutting 

 the entire gorge, as there were two epochs in the history of the 

 river when the volume was very small, so that the conditions 

 affecting erosion were somewhat similar to those illustrated by the 

 American Fall. The rate of recession of the Horseshoe Fall 

 from 1842 to 1905 is found to be 5 feet per annum, with an uncer- 

 tainty of 1 foot. For the American Fall the rate of recession 

 from 1827 to 1905 was less than 3 inches per annnm. j. b. 



4. Michigan State Geological Survey / Alfred C. Lane, 

 Director. Geological Report on Bay County, by W. F. Cooper. 

 From the Annual Report for 1905, pages 135-426, with 17 

 plates. Lansing, Michigan, 1906. — Bay County, Michigan, is 

 situated on the western and southern shore of Saginaw Bay. It 

 yields no rock outcrops, so that early surveys have given little 

 attention to its geology. The presence, however, of valuable 

 coal deposits at depths varying from 1 00 to 200 feet and more 

 has led to an extensive series of test-holes from which valuable 

 facts are obtained. The complete geological section, further, 

 is yielded by a well sunk by the North American Chemical Com- 

 pany at South Bay City, extending to a depth of about 3,500 

 feet. This reveals, below one hundred feet of Pleistocene sands 

 and clays, a series of strata from the Saginaw formation to the 

 Dundee, or Corniferous, limestone. The chart giving the care- 

 fully tabulated results of this deep drilling presents many points of 

 geological interest and importance. Mr. Cooper gives an account 

 of this section in detail, and then goes on to discuss the coal 

 formation with details as to the special layers from which workable 

 coal is obtained. The Upper Verne coal, which has had a greater 

 economic development than any other seam in Bay County, from 

 a record of two hundred and three test-holes, is found to 

 have an average thickness of nearly twenty-nine inches, and 

 taking into account the fact that in two hundred and fifty-nine 

 drill holes in the same region the seam does not occur, an 

 approximate thickness for the whole of twelve inches is inferred. 

 From this it is calculated that the total amount of coal present 

 in this seam is not far from 500,000,000 tons. The records of 

 the drill-holes, four hundred and sixty-two in number, are given 

 in detail, with maps illustrating their distribution. Other 

 chapters discuss the Quaternary, the physical geography and 

 drainage, also the economic geology, including salt and fire-clay 

 industries, and particularly the water supply. 



5. Illinois State Geological Survey, H. Foster Bain, Direc- 

 tor. Bulletin No. 3. Composition and Character of Illinois 

 Coals ; by S. W. Parr. With chapters on the distribution of 

 the Coal Beds of the State, by A. Bement, and tests of Illinois 

 Coals under steam boilei*s, by L. P. Breckenridgjs. Pp. 86, 

 with 5 plates and and 16 figures. Urbana : 1906 (University of 



