578 Dr. Richetts — On the Cause of the Glacial Period. 



the waters of the two oceans to mingle. If it has occurred to any 

 considerahle extent, it must necessarily have progressed in an increas- 

 ing ratio, unless there were any counteracting forces brought into 

 action ; for the causes which induced it would still have been in 

 operation ; sediments brought down by the Amazon and Orinoco 

 would still have been carried into the Caribbean Sea, and in much 

 greater quantities, for the Equatorial Current, meeting with no obstruc- 

 tion at the isthmus, would have been propelled, with greater celerity 

 by the trade-winds ; whilst such portions of the sediments from the 

 Mississippi, as are now carried towards Florida, would have been de- 

 posited in the Gulf, and, if the winter climate in the north increased 

 in severity, the amount of these deposits would be augmented, as 

 the frost would disintegrate the rocks more rapidly, and in a greatly 

 increased ratio, if the land was covered with glaciers. 



If the temperature of our island can be influenced in any appre- 

 ciable degree by changes of temperature affecting the Arctic Ocean, 

 the winter temperature would be lowered to an immensely greater 

 extent should any considerable volume of Equatorial water be 

 diverted across the supposed submerged isthmus, for the hottest of 

 the water would pass into the Pacific Ocean ; whilst according to the 

 size and depth of this diverted current, the Gulf Stream in the 

 Atlantic would become less and less, in consequence of the diversion 

 of that force by which it was impelled forward. 



With an indentation of Central America similar to the present, 

 there could, under no circumstances, have been the same amount of 

 warmth conveyed to such high southern latitudes by the Brazilian 

 Current as now passes to the north by the Gulf Stream, for, not only 

 the whole of the northern division of the Equatorial Current must 

 have been propelled into the Caribbean Sea, but a considerable 

 amount of the southern division also, from which, on account of the 

 obstruction caused by the south-east trade-winds, there would be 

 no method of escape southwards, and therefore it must have been 

 borne towards the north, as it is 'now by the Gulf Stream. But 

 should there have been depression of the isthmus, not only would 

 there have been, as we have seen, reduction of temperature in the 

 North Atlantic, but in the South Atlantic also ; for if it were not 

 for the obstruction caused by the isthmus, the power and extent of 

 the Brazilian Current would be much decreased, in consequence of 

 the greater portion, and that the hottest, of the southern division 

 of the Equatorial Current continuing onwards past Cape St. Boque, 

 so much of it not being deflected along the east coast of South 

 America as there is now. 



If this submergence of the isthmus has taken place, and it is the 

 necessary result of those changes which were in progress during 

 the Tertiary Period, the extension of the Gulf of Mexico northward 

 would still have continued, as it did then, far into the interior of 

 the continent; but with the deflected Equatorial Current it would 

 no longer be supplied with water of a tropical temperature, but 

 would have had the normal temperature of the latitude, and even 

 this would have been greatly diminished by the glacial coldness 

 of the waters brought down by the Mississippi and the Ohio. 



