130 - AMERICAN FERN JOURNAL 
miles from east to west, while the southern portion is 
about forty miles long from east to west and perhaps ten 
miles wide at its greatest north and south extent. The 
intervening marsh consists of the mangrove formation 
running in from indenting bays at the east and west, 
and in the middle can be waded in the dry season. Evi- 
dently the marsh is being filled in by the aid of vegeta- 
tion, and eventually the two portions of the island will 
become united by dry land. 
Topographically the island is a low-lying level plain 
from which rise sharply here and there broken ridges, 
which have a general trend from northwest to southeast. 
The ridges are composed of steeply inc’ined strata of 
more resistant rocks, the strata dipping to the east- 
northeast, so that the slopes are not generally so precipi- 
tous on that side as on the south and west. In the north- 
ern part of the island the Sierra de los Caballos reaches 
a height of about a thousand feet, and, like its companion 
ridge about three miles to the west, the Sierra de las 
Casas, it has quite precipitous slopes and is composed — “ 
mainly of a crystalline marble. ‘The Sierra de Cafiada 
mt the west nicmesin part, be the island has somewhat more 
+ wr 
reports to the contrary, 
: consists of an impure mica schist. This ite was found 
by the aneroid to lack but a few feet of the height of 
— Caballos, in the northern part of the island. . 
_ - Inland the island has a gently undulating plats of a 
ae chabiy purely subaerial erosion and reaching a height _ 
of about two hundred feet above the sea in the central © 
_ part of the island. ‘The drainage areas run in all diree- 
es radially from the central portion of the island, and 
