HILL : GEOLOGY OF THE ISTHMUS OF PANAMA. 173 



choppy waves of the Caribbean are driven into and against it by the 

 trade winds, he cannot but conclude that it owes much of its outline to 

 marine erosion. 



Manzanilla Island. — The island upon which Colon is built is now a 

 small square piece of land, which originally, before modified by man, 

 hardly projected above the water as a fringing coral reef. 1 Two thirds 

 of its present area is still a mangrove swamp lying between the sea front 

 and the mainland back of it. Much of the present terra jirma has been 

 artificially constructed by filling from the work of the railway and canal. 

 The low beach on the Caribbean side is a dead coral reef, exposed between 

 tides, and upon which the surf sweeps debris of shells, coral, tropical sea 

 beans, and other flotsam. At no other point on the island than this 

 beach is there an exposure of consolidated rock. 



The island was undoubtedly once a shallow swamp continuous with 

 the mainland, and limited on the seaward edge by the coral reef con- 

 stituting a narrow neck of land between Nava and Manzanilla Bays. 

 It is now separated from the mainland by a small short strait known as 

 Fox River, only a few yards in width, which has been clearly cut out 

 by wave erosion, destroying the neck which once connected Manzanilla 

 island to the mainland. 



Manzanilla Bay. — This is a small bight similar in outline to the larger 

 Nava Bay, but lying to the eastward of the island. Both Nava and 

 Manzanilla Bays show the same type of structure. "Where erosion is 

 taking place the surf is muddy and unpropitious for coral growth, but 

 as the surf line extends farther and farther inward the clear marine 

 water line also follows it. Hence the living coral reefs which originally 

 grew on the outer points of the harbors have followed the clear waters 

 along the interior margins of the points of the bays. 



The Isthmian Swamps. — From the steamer's deck, occasionally wide 

 flats covered with dense vegetation of plantain, sedges, and grass can be 

 seen skirting the ocean's margin, or extending inland up the flat valleys 

 between the high hills. Limon Bay is largely backed by swamps of this 

 character. With few exceptions the Panama Railway and much of the 

 canal follow these swamps for many miles into the interior. Now and 



1 The material of these reefs has been studied and published in several papers by 

 Prof. A. E. Verrill. In Proceedings of the Essex Institute (p. 323, April, 1866), he 

 states that these reefs at Colon (at that time called Aspinwall) " have essentially 

 the same features as those of Florida and the West Indies, . . . but at Panama none 

 of these forms occur, nor even any of the genera or families to which they belong, 

 with the exception of Porites." See also American Naturalist, Vol. III. p. 500, 1869. 



