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consideration of certain variable factors, such as the power of crystalli- 

 zation of the magma, diffusivity of heat, production and absorption of 

 latent heat, and undercooling, it is possible to make mathematical deter- 

 mination of the size of grain that should form at a given distance from 

 the cooling surface. Gradual change in the temperature of the margin 

 is one of the factors most difficult to estimate. It seems obvious that 

 the margins of certain even-grained dikes were near the temperature of 

 the magma at the time of crystallization. 



R. C. M. 



Temperature of the Copper Mines. By A. C. Lane. Ann. Rept., 

 Board Geol. and Biol. Surv., Michigan (1911), pp. 757-73, 

 Fig. 1. 

 The temperature at various depths in the copper mines is of practical 

 interest because of its importance in determining to what extent men 

 may work with efficiency, and of scientific interest because of its relation 

 to the circulation of mine waters and possibly to the formation of native 

 copper, as indicated by the result of recent experimentation. The mean 

 annual surface temperature in the copper region is about 39 F., that in 

 the upper levels of the mine at the depth of "no variation" between 

 43 and 44 F. Careful measurements of the rate of increase of tempera- 

 ture downward have been recently made which give an average of i° F. 

 for 105 feet descent, or in certain cases as low as i° in in feet. This 

 rate is below the normal and may be due to a number of causes. The 

 more important factors are thought to be (1) endothermal reactions 

 connected with the deposition of copper, (2) the high diffusivity of the 

 strata, permitting the free escape of heat, (3) downward absorption of 

 water carrying the cooler surface temperatures, and (4) relative exhaus- 

 tion of the internal supply of heat by the Keweenawan and earlier erup- 

 tions. 



R. C. M. 



