VARIATION OF GEOTHERMAL GRADIENT 520 



attracted attention from time to time,* mainly in connection with the exception- 

 ally low gradient of the copper country. Temperature observations from various 

 parts of Michigan were given, which will be given more fully in the Annual Report 

 of the State Geologist of Michigan for 1901. 



Characteristic figures are the following: 



In the copper country, where the mean annual air temperature is 49 degrees 

 Fahrenheit or below (38.6 degrees), the temperature increases from 43 -f- degrees 

 at 112 feet to 87 degrees at 4,900 4- feet. 



The gradient is somewhere about i degree Fahrenheit in 107 to 115 feet. In the 

 deepest mine at Ishpeming the highest temperature obtained in July, in the 

 nineteenth level, 900 feet below the surface, was 51 degrees. The mean air and 

 100-foot temperature are not far from the same, as in the copper country. 



At Cheboygan we have : Mean air temperature, 41.6 degrees ; at 408 feet (mainly 

 through drift, flow of water), 51.8 degrees ; at 1,360 feet, 61.6 degrees ; at 2,700 feet, 

 73 degrees. 



At Bay City we have from a mean air temperature of 45.4 degrees Fahrenheit 

 and temperature of first flows at 102 feet of 47 degrees a rise to 97 degrees Fahren- 

 heit at 3,455 feet. 



At Grayling, from a mean air temperature of 43.4 degrees, we have a rise to 95.9 

 degrees at 2,600 feet, the most of this in the upper part, which was drift. 



At Muskegon the mean air temperature is 46.8 degrees. The temperature of 

 numerous flowing wells from the top of the bed rock at 240 feet is 53 to 53.5 de- 

 grees. In a w 7 ell (Ryerson's salt well, abandoned, plugged at 1,200 feet) we have 

 53.2 degrees at 240 feet, 58.7 degrees at 650 feet, and 67.2 degrees at 1,150 feet. 



The facts seem to point to a difference of air and soil temperature, due to the 

 blanketing effect of snow, of half a degree to 4 degrees, according to location, and 

 a gradient in surface deposits of 1 degree in 49 feet, more or less, in shale of 1 de- 

 gree in 60, and in sandstone of 1 degree in 60 to 70 feet, while in limestone, trap 

 and the denser rocks it is 1 degree to 100 feet and more, the denser and less porous 

 rocks having greater diffusivities and lower gradients. 



Tabulation of standard results on rock diffusivitj'- seems to show that the density 

 is the most important factor in diffusivity. Contradictory observations on rock- 

 salt may be due to a diathermic effect which had not been eliminated. 



The following two papers were read and discussed together : 



ORIGIN AND DISTRIBUTION OF THE LOESS IN NORTHERN CHINA AND CENTRAL 



ASIA 



BY G. FREDERICK WRIGHT 



This paper is printed as pages 127-138 of this volume. 



* J. D. Everett, Report of Committee of British Association. 

 H. A. Wheeler, Am. Jour. Sci., vol. 32, 1886, pp. 125-13" 7 . 

 A. C. Lane, Mineral Industry, vol. 4, 1895, p. 767. 

 A. Agassiz, Am. Jour. Sci., vol. 50, 1895, p. 503. 



A. C. Lane, Am. Jour. Sci., vol. 9, 1900, p. 435, and Annual Report for 1901 to the Board of Geo- 

 logical Survey of Michigan, p. 244. 



