HILL: GEOLOGY OF THE ISTHMUS OF PANAMA. 179 
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." '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 firma has been 
artificially construeted 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 débris 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. Зо 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. 929, April, 1866), he 
states that these reefs at Colon (at that time called Aspinwall) “һауе essentially 
the same features as those of Florida and the West Indies, . . . butat Panama none 
of these forms occur, nor even any of the genera or families to whieh they belong, 
with the exception of Porites." See also American Naturalist, Vol. ILL. p. 500, 1869. 
