356 C. Davison — Effect of Glacial Period — • 



IV. — On the Effect of the Glacial Period in Changing the 



Underground Temperature Gradient. 



By Charles Davison, M.A., F.G.S. ; 



Mathematical Master at King Edward's High School, Birmingham. 



IN his recent work on " Controverted Questions of Geology " 

 (p. 159), Professor Prestwich remarks that the intense cold of 

 the Glacial period may still be perceptible in the underground 

 temperature gradient; that "to a certain depth the rate of cooling 

 is now abnormally slow, owing to the excessive refrigeration the 

 crust then underwent." The suggestion is a valuable one, and 

 I propose to test it in the present paper by estimating roughly the 

 change which the temperature gradieut may have experienced since 

 the close of the Glacial period. 



In order to avoid confusion, it may be well to state precisely the 

 meaning which is here attached to the term " temperature gradient." 

 If the temperature increase 1° F. for every 50 feet of vertical 

 descent, the gradient is taken to be 1° per 50 feet or xo° per foot, 

 but not 50 feet per degree. If the gradient in course of time were 

 to change to 1° in 60 feet, it will be said to have decreased, whereas 

 in the latter case it would be said to have increased. 



The iirst point to be determined is the rise of mean annual 

 temperature which has taken place since the Glacial period. The 

 present value of the mean annual temperature in England varies 

 between 45°'6 (Dartmoor), and 51°-9 (Helston), the average over 

 the whole country being 49°'5 F.* With regard to the Glacial 

 period, we are concerned, not so much with the mean annual 

 temperature of the air at that time, as, in glaciated districts, with 

 that at the base of the ice- sheet ; and this, it is probable, underwent 

 but little change during the whole time the ice-sheet lasted, however 

 intense the cold at its surface may have been. For, if the thickness 

 of the ice exceeded a certain limit, the temperature at the bottom, 

 owing to the rise of the isogeotherms, would be maintained at the 

 melting-point corresponding to the pressure of the overlying ice. 

 With a mean temperature of 0° F. at the upper surface of the ice, 

 and a gradient in the rock below of one degree in 60 feet, Mr. 

 Fisher finds this limiting thickness to be 714: feet.* With a gradient 

 of one degree in 50 feet, the limit would be 596 feet. This may, 

 however, be an under estimate, for it does not take the depth of the 

 "surface of invariable temperature" into account. Moi'eover, while 

 the temperature at the depth of this surface is in rock the mean 

 annual temperature of the locality, it may be less in the case of an 

 ice-sheet whose temperature can never rise above the melting-point, 

 and is therefore incapable of following the oscillations of the 

 external temperature above that point. The effect of the omissions 

 is not, however, very sei'ious, involving indeed in some cases an 

 increase in the above estimate, though not, it may be, a great one. 



1 This is the average of the annual means at 57 stations given by Dr. A. Buchan 

 in his great work on "Atmospheric Circulation," "Challenger" Reports, Physics 

 Chemistrv, vol. ii, pp. 196-199. 



■ Phil.' Mag., vol. vii, 1879, p. 385. 



