170 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 



experience in observing the phenomena I have never seen such an ad- 

 vanced example of a decaying region. 



Topography of the Submarine Bench. — In considering the Isthmian 

 topography it is necessary to extend our profile seaward in both direc- 

 tions in order to understand the true contour of tbe cross section, for on 

 both sides there exists a shallow submarine bench, which must have been 

 a continuation of the land in these directions. The submarine bench of 

 the Caribbean is a gradually sloping plain for many miles northward of 

 Colon until the depth of 500 fathoms is reached, presenting a flattened 

 gradient. Then the profile makes a great plunge from 600 to 1,900 

 fathoms, presenting a submarine escarpment which, although of great 

 height, is not so abrupt as that of the Pacific. 



On the Pacific side the waters of Panama Bay are so shallow that 

 their average deepening does not exceed one fathom per mile, until 

 the 100 fathom line is reached, nearly 100 miles south of Panama city. 

 This line will almost connect Cocalita Point and Cape Mala, the two 

 points which mark the entrance to the Gulf. Here, however, almost 

 coincident with the abrupt Pacific coasts, there is a gigantic submarine 

 escarpment plunging off into the Pacific to the depth of 1,700 fathoms 

 or more. These features we have added to the profile of the land on 

 Plate IV. Figure 2. 



The submarine topography of Panama Bay is also of deepest interest 

 in interpreting the history of the former magnitude and decay of the 

 land. Xot only do the numerous islands stand as monuments to its de- 

 cay, but the floor of the bay itself reveals a remarkable surface. I have 

 carefully plotted upon the map, as shown in Plate IV., the submarine 

 contours of Panama Bay. The results clearly show the presence of 

 a topography remarkably similar to that of the land, including several 

 deep arterial channels which may have been submerged river valleys. 

 This submarine topography may represent the continuation of the larger 

 rivers many miles out into the region now covered by the Pacific waters, 

 and strongly suggests that the ramifying systems of drainage now visible 

 are but remnants of the headwater portion of what were once far more 

 extensive streams. 



There are two great factors in the destruction of the land of the Isth- 

 mian region : 1st, the excessive rainfall ; and, 2d, the marine erosion of 

 the waters of the Caribbean and Pacific. 



Tin- rainfall of the Isthmian region, with the accompanying erosion, 

 is excessive, exceeding any other upon the western hemisphere. For the 

 year 1S9-4, the one preceding my visit, according to the measurements 



