SURFACE STRUCTURE OF THE UNITED STATES. 147

 

and Pamlico Sounds. On the western coast are
the Gulf of Georgia and the fine harbor of the
Bay of San Francisco.

The Atlantic shores slope gently toward the
ocean; the Pacific shores are abrupt.

383. Islands—The islands of the Atlantic coast
are of three distinct classes: those north of Cape
Cod are, for the most part, rocky, and are de-
tached portions of the mainland; those south of

Cape Cod are generally low and sandy, and are,

for the most part, of fluvio-marine formation ; those
off the coast of Florida are of mangrove forma-
tion. On the Pacific coast are the Santa Barbara
Islands, a rocky group south-west of California;
and Vancouver Island, north-west of Washington.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Fig. 123, View on the Ooast of Mount Desert Island, Maine.

384, Mangrove Islands.—Mangrove trees grow
in dense jungles, on low muddy shores, in tropical
regions. From both trunks and branches the
trees throw out air-roots, which spread so as to
cover the adjoining spaces in an almost intermin-
able network of roots and branches. The area of
surface covered by the trees is still further in-
creased by the curious property which the seeds
possess of sprouting while on the tree, subse-
quently floating away, and afterward affixing

themselves to the bottom of the jungle, to form

new growths. In this way, the trees form man-
grove islands, which at first are not true islands,
the trees simply standing above the water by
means of their intertwined roots. In course of
time, however, sediment, collecting between the
roots of the trees, forms islands. These islands
are common in the shallow water off the coasts
of Florida.

385. Coral Reefs of Florida.—The peninsula
of Florida, south of the northern extremity of the

Everglades, and probably as far north on the

17

   

 

 

eastern coast as St. Augustine, is, according to
Agassiz, a species of coral formation, formed,
however, under different conditions than are the
coral islands of the Pacific.

 

 

Fig, 126. Everglades, Reefs, and Keys of Florida, (LeConte.)

Figure 124 is a map of Florida with its reefs and keys,
Figure 125, is a section along the line A.A. In Fig. 124
the line a a, shows what was at one time the limit of the
southern coast of Florida. 6}, is the present limit of the
southern coast. cc, are the keys, which are low islands.
dd, is the growing coral reef. e, is the Everglades, dotted
with islands, called hunmocks. Between cc, and d d,_is
the ship channel. Outside the growing coral reef dd, are
the profound depths of the Gulf Stream G. 8.

The growth of the reef-formations is explained by
LeConte as follows (Fig. 125): a, was at one time the limit
of the southern coast of Florida. 8, is the present southern
coast, which at one time was a coral reef like d. Upon 8,
a line of coral islands gradually formed connecting it
with the old southern coast a. The ship channel between
a, and }, gradually filled up and formed the Everglades e.
Meanwhile, another reef formed, in the position of the
present keys, ¢, the ship channel being between 5b, and c.
This reef has now grown to be a line of coral islands,
and the ship channel, between }, and ¢, converted into
shoals and mud flats, f. The present ship channel is
between c,and d. In course of time the southern coast
will extend to the present line of keys, c, and the shoal
water f, will become another Everglades. Outside the
present keys c, another coral reef d, is growing, to which
the coast will ultimately extend, and which will mark
the limit of the formation, owing to the deep waters