MICHIGAN ACADEMY OF SCIENCE. 101 



to have been built up by outwashed sediments, may in part represent crustal 

 materials left when the Atlantic rent opened, instead of being wholly the 

 result of upheaval of the ocean floor. 



Clearly, in these contours of the northern ocean bottom we are confronted 

 with changes of profound significance and to single out any particular depth 

 curve and apply it as a test of interpretations reaching back to the dawn 

 of Tertiary time would be improper. 



The Gulf of Mexico receives the silt from an area approximately four 

 times its own and its filling up must be correspondingly rapid. When the 

 Great Lakes emptied by way of the Mississippi river the drainage area was 

 considerably greater, probably five times that of the Gulf, and yet, the 

 evidences of filling are nowhere near what they are about the borders of the 

 North Atlantic. There is still a small 2000 fathom area left in the west- 

 ern part and the 1000 fathom outline includes perhaps a half of the whole. 



There is just one feature in connection with these depths in the Gulf that 

 I wish to mention particularly and that is the relation of Florida to the coast 

 of Yucatan and Honduras. It will be recalled that in the replacement of 

 middle America I place the peninsula of Yucatan to the west of that of 

 Florida with a gap between about as wide as the latter peninsula. Reference 

 to the bathymetrical charts discloses the 100 fathom curve about that dis- 

 tance from the west coast of Florida, but on the east coast of Yucatan it 

 is close inshore and the bottom drops rapidly clear down to 2300 fathoms 

 (2.6 miles). Here is a case where it would seem that the west Florida 

 shoal corresponds to the missing area, and the deep off the Yucatan coast 

 would harmonize with the scheme of matching together. But after all 

 this is a mere approximation; the depths involved are not great enough and 

 moreover we no sooner begin such comparisons than we find insuperable 

 obstacles to their continuance elsewhere. Just as in the matching together 

 of Africa and the Americas we find too little land at the borders and have 

 to leave spaces between, so here in the attempted use of the submarine 

 contours, 100 fathoms or greater, we find too much and have to lap them by, 

 sometimes more, sometimes less. But this difference is vital — in the one 

 case we have coast lines to guide us, in the other there are no guides what- 

 ever. 



Of all South America the greater part of the disintegrated material is 

 dumped upon the eastern coast to the north of the Amazon and the south 

 of La Plata. Along this coast the submarine border must be changing both 

 rapidly and irregularly and where a given depth still maintains a fairly 

 regular distance from the shore, as is doubtless true over great stretches in 

 spite of undoubted disturbing agencies, that fact may require special ex- 

 planation. 



Touching next upon vertical crustal movements, it is to be observed that 

 for purposes of movement the submarine margin is inseparable from the 

 rest of the continental structure. Any vertical movements which may 

 effect the strand maj' also affect the hundred fathom depth although, as 

 Shaler' ])ointed out, the changes may possibly be in op]50site directions 

 under the sea and upon the land. L^^nder the theory advanced by Chamberlin 

 and Salisbury'-, Shaler's concept of sinking sea bottom is extended to imply 

 pressure towards the continents, shearing and deformation with creeping of 

 the soft sediments of the continental shelf. The evidence which is being 



'Shaler, N. S., "On the Nature of the Movements Iiivolvetl in the Changes of Level of Shore 

 Lines." Proc. Boston Soc. Nat. Hist. vol. xii, 1868-69, pp. 128-136. 

 ^Chamberlain and Salisbury. "Geology" 1906 vol. iii, pp. 526-530. 



