124 GEOLOGY. 



would have been attained, but the crumpling of the continental surfaces would 

 be left unexplained. 



4. If the sub-oceanic sectors sank first, for the reasons previously given, and, 

 by virtue of their superior gravity, wedged aside and squeezed up the continental 

 sectors between them, the continental surfaces would doubtless be warped and 

 crumpled, and great reliefs and surface distortions be produced simultaneously. 

 But would the proportions of lateral thrust and continental elevation accord with 

 the facts? While it is necessary to recognize that no estimate of the total amount 

 of lateral thrust involved in the mountain folding that has actually taken place 

 is more than roughly approximate, we have previously taken 100 miles as a round 

 figure representing the apparent order of magnitude of the crustal shortening 

 due to folding since the pre-Cambrian times, on an entire circumference (Vol. I, 

 p. 551). If, without change of volume, the continental sectors were squeezed 

 proportionately throughout their mass by the sinking oceanic sectors, so that 

 their surface breadths were reduced 100 miles, in the aggregate, on each circum- 

 ference, they would be protruded several times as high as they actually are. It 

 is obvious that this is not the whole of the actual case, though it seems to be a 

 step toward it. 



5. We have heretofore recognized the possibility that owing to the original 

 distribution of heat in a self-compressing earth, the temperature may be still 

 rising in the outer 800 miles or so of the earth, while it is sinking below, and that 

 this may bring to bear special crowding in the outer parts (Vol. I, pp. 564-568). 

 It may be, also, that crystallization has limits determined by pressure and heat, 

 and that only a certain outer zone is crystalline while the central mass is in a 

 miscible state, or in some unknown state of superior density. If this were true 

 the outward distribution of heat and slightly increasing gravitative pressure 

 might lead to the encroachment of this central mass upon the crystalline zone, 

 by transformation from the crystalline to the miscible or other central state. 

 This would result in a crowding of the crystalline zone on itself in the effort at 

 adjustment to the shrinking central mass, in a manner analogous to the familiar 

 crowding of the superficial crust on the shrinking subcrust. If in these, or in 

 other and unrecognized ways, the outer parts of the sub-oceanic sectors be sup- 

 posed to thrust the outer parts of the continental sectors relatively more than the 

 central parts crowd one another, surface distortion, greater than in the previous 

 case in proportion to the continental elevation, would result, and a nearer 

 approach to the true case would be attained, but the proportions would still be 

 wide of the true ones. But the kind of effect that would be produced in this way, 

 normally, seems to suggest the source of a needed factor. If a segment, say 800 

 miles thick, were thrust against another segment of like thickness so as to shorten 

 the latter 1 mile, with only negligible change of volume, the surface of the latter 

 segment must be raised a mile over an area 800 miles in width, or its equivalent. 

 Some change of volume by compression would be inevitable, but for the general 

 conception here sought it may be neglected. Such crowding of thick seg- 

 ments seems to be of a kind suited to form plateaus, for the effect would prob- 

 ably be a swell or uplift whose breadth was more or less comparable to the thick- 



