HYPOTHESIS OF CONTINENTAL STRUCTURE 391 



area becomes a basin of marine sedimentation. Such was tbe coastline of 

 the Paleozoic mediterranean in the Appalachian province. 



4. It is deemed probable that the continental elements differ in density 

 among themselves and also in comparison with the suboceanic masses, this 

 difference being an original character in the sense that it has distin- 

 guished the individual masses as far back in geologic history as the record 

 of erosion and sedimentation is intelligible, and probably much farther 

 l^ack. 



5. There have been horizontal movements as well as movements in a 

 vertical direction. The horizontal displacements are of notable magni- 

 tude, and their effects are seen in the schistose structure of the once deep- 

 seated rocks and in the overthrust and folded structures of the more 

 superficial strata. 



6. An effect of horizontal movements has been to crowd the continental 

 elements together, and consequently to cause a certain amount of mashing 

 in their deeper-seated portions. Just how the shortening may be distrib- 

 uted below the superficial crust which we can observe, we do not know: 

 but from the universal occurrence of schistosity in rocks exposed by pro- 

 found erosion, it seems probable that the compression is not far from 

 uniform. On the other hand, at the surface there are diversities of 

 structure and attitude which have served to concentrate the effects of 

 horizontal displacement in certain zones, namely, in zones where sedi- 

 mentary rocks had accumulated to considerable thickness. 



7. The concentration of the effects of horizontal thrust in zones of 

 sedimentary rocks is due to: (a) stratification, which determines the 

 arrangement of material in extended sheets that are capable of moving 

 over one another, and consequently of folding with comparative ease; 

 (b) the deflection of the strata from a strictly fiat attitude in such man- 

 ner that initial lines of flexure are established, on which primary anti- 



■ clines and synclines are later developed.* This initial deflection results 

 from unequal subsidence during the process of deposition. 



8. Eecalling that coastlines are established along the monoclinal flex- 

 ure which joins a rising with a sinking continental element, and that 

 sediments accumulate to greatest thicknesses usually in a zone parallel to 

 the coast, we see that zones of folding are commonly coastal zones. Since 

 progressive subsidence results in the development of initial dips in lines 

 essentially parallel to the coast, and since initial dips determine the axial 

 directions of folds during the next epoch of deformation by horizontal 

 stress, it follows that the axial directions of folds conform to the general 



* Mechanics of Appalachian Structure. Thirteenth Ann. Report U. S. Geol. Survey, 

 pp. 2.53-258. 



