Level-qf -no- Strain in a Cooling Globe. 213 



Colorado Plateaus. — The contractional hypothesis is equally 

 incapable of accounting for the elevation of the great mono- 

 clinal plateaus lying to the west of the Rockies. Indeed this 

 has been insisted upon by American geologists, notably by 

 Dutton and Gilbert, before the discovery of the level-of-no- 

 strain. It is, therefore, unnecessary for me to dwell upon 

 these arguments here, except to say that by this discovery 

 the difficulties are increased beyond all possibility of ex- 

 planation. 



Effects of Contraction on the Shell underlying the Shell-of- 

 Compression. — The bulk of the contracting portion of the 

 earth's crust, even if the whole crust be estimated at not 

 more than 30 miles thick, is so much in excess of the shell-of- 

 compression, in addition to the much larger underlying con- 

 tracting envelope, that I fully expect some reverse theory of 

 mountain-building may be based upon it. Mr. Davison has 

 already suggested that the ocean-basins are due to the locali- 

 zation of this contraction. Let me examine the problem in a 

 fair spirit, and as fully as the limits of this paper will allow. 

 In discussing the effects of contraction on the rocks of the 

 crust underlying the shell-of-compression, I have said, " Prac- 

 tically tension could not take place, as the superincumbent 

 strata would by vertical compression elongate the rocks at 

 the zone of greatest contraction to fill the vacuities that 

 otherwise would be created."* 



The depth of the zone at which compressive-extension com- 

 mences will differ according to the nature and crushing 

 strength of the rocks : but we may safely assume that all 

 known rocks will be plastic under the superincumbent pressure 

 at a depth of 10 miles f. It is, therefore, possible that some- 

 where in the solid crust between that depth and the surface, 

 cavities might be formed by secular contraction, and shearing 

 and faulting take place by the subsidence of overlying masses 

 of crust into the plastic medium below. Irregularities of the 

 surface of the sphere might arise in this way, as well as by 

 inequalities in the amount and mode of shrinkage of the whole 

 of the underlying contracting body or shell. It needs, how- 

 ever, but little consideration to see that such orographic 

 changes of the surface would tend generally in the same 

 direction; depressions once commenced would increase in 

 depth by a continuation of the contraction which initiated 

 them; while such elevations as might originate in the much 

 smaller shell-of-compression would tend to increase in height 

 and in number also. But while the agencies invoked in our 



* ' Origin of Mountain-Kanges/ p. 125. 

 t Ibid. .91. 

 Phil. Mag. g. 5. Vol. 25. No. 154. March 1888. Q 



