172 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 



well known. While the ebb and flow is only about 27 inches at Colon, 

 the greatest rise and fall at Panama is 21 feet. The impact of this great 

 wave as it beats against the Pacific coast creates a powerful erosive 

 force. The undertow and flow of the tide has also great effect in remov- 

 ing and distributing coastal debris. 1 All of the islands as well as most 

 of the mainland of the Pacific side exhibit, just above low tide level, ver- 

 tical or concave cliffs corresponding in height to the difference between 

 extreme high and low tides which are being constantly undermined by 

 the tidal action. This tidal cliff is a striking feature of all the sketches 

 in this paper of the Pacific coast and islands in the Bay of Panama. 

 Ko one glancing at the latter and comparing them with the mainland 

 topography will doubt that they are being slowly but surely destroyed 

 by marine erosion, and that this erosion severed them from the main- 

 land of which they were undoubtedly once a part. And it would require 

 no stretch of the imagination to conceive that the great Gulf of Panama 

 itself may have been cut out in this manner by the progressive indenta- 

 tion of the Pacific Ocean. (See Plate III.) 



PART II. 

 Geology of the Continental Section, Colon to Panama. 



The Caribbean Coast. 



From the Puerto Bello headlands to Colon, the coastal topography 

 consists of rugged pointed hills rising close from the sea, varying in 

 height, but not exceeding 700 feet. These are separated by deep XT- 

 shaped valleys cut down to the level of the sea and exteuding far inland. 

 Occasionally small beaches or even outlying islets give variety to the 

 rugged coast. Over the hills and valleys spreads the great veil of tropi- 

 cal vegetation concealing all the minor details of the immediate surface. 



Colon Harbor. — The quadrangular Nava Bay, which is a square cut 

 inlet into the coast line, is terminated at the sea by the Toro and Man- 

 zanilla points, the latter being the northeast corner of the small island 

 upon which Colon is situated. This oblong harbor has no important 

 stream entering it. As one watches the constant force with which the 



1 The jetties constructed recently at Galveston, Texas, have so chanced the 

 direction and action of the surf that the beach of Galveston Island is being rapidly 

 washed away. This action is taking place from marine currents, entirely away 

 from the influence of any streams or currents from the land. 



