V 

 Prof. Spencer — The Pacific Ocean and Gulf of Mexico. 307 



that I furnish some advance notes concerning recently observed 

 phenomena which greatly strengthen the theory of stupendous 

 changes of level in the Pleistocene period. Many months must 

 elapse before I shall be able to complete the studies for publication, 

 so that my papers on Cuba, Jamaica, and Mexico shall be published. 

 It may be said that the date of the recent continental elevations are 

 based on the extent of the erosi(m of the Miocene and later deposits. 

 In the south-eastern states but little of the Upper Miocene (American 

 equivalent) has been preserved from erosion. In Cuba and Jamaica 

 the Miocene limestones have suffered enormous degradation, so that 

 probably only the lower strata remain, yet these are of immense 

 thickness, although they are often completely incised by great valleys. 

 The same phenomena hold good for regions south of the Mexican 

 Gulf. The determination of the ages of the beds is based upon 

 palfeontological as well as stratigrapliical and lithological evidence. 

 The lists of fossils will be submitted in forthcoming papers. In the 

 southern states these deposits are not disturbed to any great extent, 

 but beyond they are everywhere upturned and often occur at high 

 angles. Overlying and occupying portions of the great valleys, 

 there are the widespread loams and gravels of the Lafayette 

 formation of the southern states, which extend also over the coastal 

 plains of Mexico. On the islands, apparently, the same formation 

 occurs, but in part represented by marls, with some included 

 gravel (the Matanzas formation), and these contain fossils of mostly 

 modern species. With the oscillations of land and sea. the perfect 

 synchronism between the eastern and western beds is not claimed, 

 but only the general equivalency, as the continental movements 

 passed from east to west and back again. These Lafayette and 

 Matanzas beds have a remarkably wide distribution, and present' 

 a wonderful degree of uniformity, so that the geology of the vast 

 region is greatly simplified. These formations have been pro- 

 visionally regarded as belonging to about the close of the Pliocene 

 period, and whether a little earlier or a little later is of no material 

 difference. It is the sequence of events which forms the importance 

 of the study, for the palseontological boundaries are somewhat 

 ill defined at present. The surfaces of these Lafayette and Matanzas' 

 accumulations have been greatly denuded, so that in many places 

 they are found only in fragments, which have nevertheless been 

 most important in the investigations concerning the ''Reconstruction 

 of the Antillean Continent." In these valleys, again, we find later 

 mid-Pleistocene formations which have been traced to the regions 

 of northern drift accumulations. The great continental elevation 

 reached its culmination in the post-Lafayette or early Pleistocene 

 epoch, so far as the eastern part of America is concerned. Bat at 

 that time I have recently found upon the Tehuantepeo isthmus, 

 where the drainage of the eastern plateau was hypothesized to have 

 passed to the Pacific Ocean, that the country was so low as not toi 

 permit the formation of deep valleys such as were being formed 

 to the east, and are now drowned beneath the sea or buried along 

 the Atlantic or Gulf coast. 



