166 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 



beside the Caribbean shore of Colombia, and of other known high peaks 

 hke Orizaba and St. EUas adjacent to the Pacific shore hues of our own 

 continent. 



So grand a subsidence and elevation as this must surely have left 

 some record in tlie present submarine topogTa2;)hy of the whole region, 

 and we cannot contemplate maps of such phenomena without coming to 

 some very definite opinions which will be elucidated in Part Y. ^Neither 

 could the maximum of elevation have failed to expand the areas of tlie 

 Antillean lands, and to unite many of the islands or even the mainland 

 together, especially had their geographic areas been greater then than 

 now, which hypothesis seems tenable. 



The amplitude of the Bowden subsidence could hardly have exceeded, 

 if it reached, 3,000 feet (500 fathoms). The data for this conclusion are 

 as follows : — The upper and lower limits of the deposition beds of this 

 epoch are now practically visible. While they have, no doubt, been 

 attenuated by erosion and their thickness has not been finally measured, 

 they do not exceed 500 feet in thickness. The lithologic and sedimental 

 character of the beds indicate a rapid gradation from off-shore to moderate 

 oceanic depths. The fundamental beds are gravels, much water worn, 

 embedded in marl, but contain no plant remains or other indications of 

 ultra shallow deposition, although land shells do occur sparingly in them. 

 The fauna could not have lived at depths of over 500 fathoms. 



The presence of nearly four hundred species of Mollusca in a single 

 horizon of one of the basement beds at first suggests immediate littoral 

 conditions. Their mode of occurrence in the gravel, however, indicates 

 slightly deeper or off-shore conditions or origin, for had this gravel been 

 near the beach line the delicate shells w'ould have been ground and 

 broken into breccia by wave action, and it is probable that they were 

 too deep to be influenced by any such action. The mollusca are such as 

 live at present at depths of less than 100 fathoms. The several species 

 of Foraminifera, Bryozoa, and corals, especially the last, which are all 

 simple non-reef-building species of the type which Pourtales has men- 

 tioned ^ as liaving probably lived at an average depth of 450 fathoms, 

 indicate abyssal or continental deposition. Tiiese facts indicate that the 

 basal Bowden beds, now exposed at sea level, were slightly below sea 

 level at the beginning of the Bowden subsidence, and that the amount 

 of the depth must be subtracted from the total thickness of the Bowden 

 to ascertain the true submergence, which could not have exceeded 500 

 fathoms. 



1 Cited by Agassiz, "Three Cruises of the Blake," Vol. II. p. 19. 



