204 bulletin: museum of comparative zoology. 



over the areas, richly studded with remains of its animal life, but so 

 different from similar life on the Pacific side that not a single fossil 

 is common to the sediments of the two oceans. 



The composition of these Lower Cretaceous beds clearly exhibits the 

 fact that this subsidence deepened towards the southeast. Twenty-five 

 thousand feet of limestones in eastern Mexico near the Tropic of Cancer, 

 as seen by McGee and the writer, testify to the fact that it there equalled 

 more than 6,000 fathoms, or as much as some of the greatest depressions 

 yet discovered in the oceans, and yet there is no evidence in the Creta- 

 ceous faunas that the Pacific barrer was broken. 



What happened in the Windward regions during Lower Cretaceous 

 time can be only hypothetically conceived. Such a subsidence increas- 

 ing southeastward may have been at least so far reaching in its effect as 

 to affect and submerge the hypothetical Jurassic bridge of the Windward 

 region. If so, the Windward barrier to the eastward was crossed by 

 Atlantic waters, which probably came in across the north side of South- 

 ern Florida, which until very recent time has been Wes^ndian in its 

 relations. The absence of any known Lower Cretaceous fossils in the 

 Antillean region suggests that a large land area may have existed during 

 this epoch, composed of south Floridian, Eahaman, Antillean, and Wind- 

 ward lands. 



The faunas of Trinidad and the northern regions of South America 

 show that in Lower Cretaceous time the Atlantic waters were overcom- 

 ing the southern end of the Jurassic Windward bridge, if it ever existed, 

 and that the Atlantic littoral faunas were encroaching upon the present 

 eastern Caribbean area, which had lost or was losing its connection with 

 the Pacific. 



In Middle Cretaceous time there was a considerable movement in the 

 northern Gulf region, causing the land to emerge and the shore line to 

 recede from southwestern Kansas to east central Texas. We have ab- 

 solute record of the extent of this movement in the known migration of 

 the line of the Dakota littoral. Inasmuch as all of the sedimental evi- 

 dences in the littoral formations of our Coastal Plain are only the margi- 

 nal phenomena of oscillations which probably had their greatest amplitude 

 to the southward, the effects of this Mid-Cretaceous movement on the 

 tropical regions must have been great. It was one of those oscillations 

 which, had it occurred in later geologic time, would have been of con- 

 siderable importance, but its effects and extent are completely obliterated 

 by the grander changes which preceded and followed it. It is probable 

 that a Central American land bridge connecting the continents via the 



