212 Mr. T. M. Reade on the Secular Cooling of 



omitted reference to the formula of Clausius, Van der Waals, 

 and Sarrau. It appears to us that the relations of a and b in 

 our formula to the volume is by no means so simple as that 

 expressed by their formulas. 



XXIII. Secular Cooling of the Earth in relation to Mountain- 

 Building. By T. Mellaed Reade, C.E., F.G.S.* 



IN Chapter XI. of my ' Origin of Mountain- Ranges ' I 

 attempted to show that the effects of secular cooling on 

 the earth's crust would not be that assumed by what is now 

 called the " Contraction theory " of Mountain-formation. 



But, first, it will be well to state what this theory is. 

 Roughly speaking, then, the earth in its present state is con- 

 sidered to be divided into two parts ; a cooled solid outer crust 

 which does not contract, and a heated nucleus which dimi- 

 nishes in volume as it parts with its heat. From this it- 

 results that the hard crust, in fitting itself from time to time 

 to the shrinking nucleus, throws up the ridges of the earth's 

 surface called Mountain-ranges. This, in its crude form, is 

 the theory which is usually presented to the reader. 



A very little consideration will serve to show that the 

 effects of the cooling of a body like the earth will be very 

 much more complex. 



If w T e assume the crust to be a hard shell, say 30 miles 

 thick, with a temperature of 50° at the outer surface, and 

 3050° at the inner surface, we shall find that the circum- 

 ferential contraction of the inner surface of the shell in 

 cooling will be much more relatively than the radial contrac- 

 tion of the earth from the centre to this zone ; consequently 

 the hard crust will at this zone be not in a state of compres- 

 sion, but one of tension or stretching. 



By means of a diagram (plate 18) I have shown that an 

 outer portion or shell of this hard crust, of very limited depth, 

 will be in a state of compression ; but by far the larger 

 volume of the crust will constitute a " shell of contraction," 

 where all the rocks constituting it will tend to stretch along 

 circumferential lines. This stretching would end in the frac- 

 ture of the shell were it not for the weight of the superimposed 

 mass, which ensures continuity by what I have called " com- 

 pressive extension." 



It is unnecessary for me to go into further details as the 

 reader can find the full particulars in the chapter referred tof; 

 excepting that I would point out that the external cooling 

 portion of the earth is shown to.be divisible into a thin outer 



* Communicated by the Author. 



t 'Origin of Mountain-Ranges/ pp. 121-128. 



