HILL: GEOLOGY OF JAMAICA. 199 
diastrophism, orogeny, and physical geography, must all be considered. 
The past and present distribution of life of all orders, both land and 
marine, their paleontologie history, the part which the débris of their 
skeletons, extracted from the liquid sea, has played in accumulating 
strata upon the submerged slopes and bottoms or building coral reefs 
are also important factors. The possibilities of the great Equatorial 
current and Gulf Stream, as carriers of sediment, corrosive agents, and 
the effect on the distribution of lime-making organisms is also a most 
important consideration. 
There are several, important conditions which make all attempts at 
final interpretation of Antillean and Tropical Americán history more or 
less hypothetical. The first of these is the fact that the submarine con- 
figuration suggests that large areas of land now submerged may have 
existed, not only in the immediate basins of the American Mediterranean, 
but also in the Atlantic and Pacific waters off the present continental 
borders. These submerged areas are now so covered with accumulations 
of organic and oceanic débris that their geologic composition can never 
be.approximated even by soundings. Much of the older sedimentary 
strata of the present land masses have also been as completely concealed 
by burial beneath vast accumulations of volcanic ejecta, especially in 
Central America, the southern end of the Mexican Plateau, and the 
Caribbee Islands. 
Notwithstanding the incompleteness of the record, the general con- 
figuration, and the geologic structure and paleontology of the land arcas, 
and the distribution of the present life of the land and sea, afford much 
data of a fragmentary nature which can be so placed together as to throw 
some light upon the geologie evolution of the region. The discussion of 
the biologic and hydrographic phases of the question must be left to 
others, and in this chapter only the testimony of the structural geology 
and the configuration will be discussed. 
The geologic composition and arrangement of the rock sheets relative 
to one another record in a manner changes of level, relative depth of 
deposition, and position of land areas. Variations in the physical and 
chemical composition of strata, when traced over wide areas, enable us to 
judge with a degree of conjecture the location of the land from which 
they were derived or the extent of their basins of deposition, and to con- 
struct hypotheses of former bathymetric variations. Fentures of land 
configuration by which events of elevation, degradation, and subsidence 
can be traced, should present harmonious and parallel conclusions 
with the interpretation of the fossils and the strata. Many writers of 
