GEOLOGY— CHAMBEKLIN. 175 



For the purposes of study, the earth may be conceived as formed 

 of two sets of great sectors, or pyramids, having their bases at the 

 surface and their apexes at the center of the earth. One set of these 

 have as their bases the several ocean bottoms, and the other set the 

 several continental platforms. If, as a first step of treatment, these 

 be taken as units of action and it be recalled that they may be 

 affected by shrinkage in all portions, but presumably in different 

 degrees at different depths, certain hypothetical conceptions may be 

 tested and either approved or rejected. 



(i) If the sub-oceanic and continental sectors be supposed to 

 have all shrunk in equable proportions in every part and simultane- 

 ously, the sectors would settle down each within its own limits and 

 no unequal crowding would follow and hence no deformation. This 

 conception is excluded by the facts of the case. 



(2) If the outer parts of all the sectors shrank throughout propor- 

 tionately more than the inner parts, each sector could not only have 

 settled down within its own space, but could have suffered some 

 teusional Assuring and faulting without involving lateral thrust or 

 folding. This is also excluded by the facts of the case. 



(3) If the sectors beneath the oceans simply contracted radially 

 more than the continental sectors to the extent of about 3 miles, the 

 existent continental reliefs might have been attained, but the phe- 

 nomena of folding and overthrust would be left unexplained. 



(4) If the sub-oceanic sectors sank first, as they should for the 

 reasons previously given, and if by virtue of their superior gravity 

 they wedged aside and squeezed up (relatively) the continental sectors 

 between them, the continental surfaces would doubtless be warped 

 and crumpled, and so the great reliefs, attended by some surface dis- 

 tortions, might be produced at the same time ; but the question of 

 the proportions between the degree of lateral thrust and of continental 

 elevation at once arises. While no existing estimate of the total 

 amount of lateral thrust expressed in folding and overthrust faults 

 can be regarded as more than roughly approximate, the probable 

 errors are not sufficiently great to vitiate the testing of this concep- 

 tion. If for this purpose we take the estimate of 100 miles shorten- 

 ing of the crust on a total circumference as the result of the folding 

 since Cambrian times, it may be readily seen that if the continental 

 sectors were squeezed proportionately throughout their mass so that 

 their aggregate surface breadths were reduced by this amount, these 

 sectors would be protruded several times as high as they actually are. 



(5) If, however, the crowding of the master sectors were greater 

 in the outer part than in the more interior portions, a greater amount 

 of surface deformation in proportion to the continental protrusion 



