152 



ON THE OEIGIN OF MOUNTAIN RANGES. 



lation that if the temperature of solidification were 7,000° 

 Fah., our globe would have radially contracted from the 

 time of solidification to the present time 1'9 miles, and con- 

 tends that, owing to circumforential shrinkage, witli a 

 radial contraction of 2'04 miles, the crust at a do[)th of one 

 mile would undergo no compression. 



Both tliose authors, therefore, reject the contraction hypo- 

 thesis on physical grounds, and contend that to produce the 

 known inequalities of the earth's surface the contraction 

 along a radius from the earth's centre, instead of being only 

 1'9 miles, ought to be from 30 to 40 miles. 



But if the compi'cssion which is an observed fact on the 

 earth's surface has not been produced by secular contivac- 

 tion, it must be due to expansion of the undoi'lay(!rs of the 

 earth's crust. Both authors, therefore, base their hypo- 

 theses, in part at least, on such expansion. Bat they account 

 for this expansion in different ways. 



Mr. Fisher's hypothesis requires a liquid iniicr-strattim 

 between the solid crust and the solid core. He suggests 

 that fissures may be formed in the lower part of the ci'ust, 

 and maybe injected with liquid from the fluid inter-stratum. 

 The upper part of these fissures would bo filled with gase- 

 ous water-substance at a pressure of 10,066 tons to the 

 square foot. The enormous pressure of this gas would tend 

 to widen the fissure and compress the rocks on either side 

 of it. " When the rent reached the surface, the vapour 

 would rush forth and be followed by the magma itself, now 

 appearing as lava ; and tlius a volcano wtnild bo established. 

 But this would be an exceptional occurrence. It would 

 only be here and there that the vapour would escape at the 

 Burface, because its doing so at one point would relievo the 

 internal pressure for a long distance." " The quantity of 

 igneous rock injected into the fissures would go to increase 



