330 TRANSACTIONS OF THE CANADIAN INSTITUTE. [VOL. V> 



A similar plateau connects Jamaica with Haiti. This is generally- 

 submerged to a depth of 5,000 feet. However, it is dissected with 

 channels reaching down to 7,000 feet, and there are small basins 

 probably connected, so as to form a channel extending to a depth of 

 over 9,600, or more than 5,000 feet below the surface of the drowned 

 plateau (see Map, page 328). 



While the soundings are very numerous in many localities, they have 

 not been made in directions for discovering the valley-like channels ; 

 nevertheless, many have been more or less revealed, but for the fuller 

 exposition, the soundings should often be taken along other lines and 

 closer together. In off-coast soundings, while the depths are accurately 

 determinable, there may be errors of locations on the charts of two or 

 four miles which may give rise to the impression of the occurrence of 

 deep holes, where there is no reason for other depressions than portions 

 of channels. However there are very few of these small basins sug- 

 gested within the Antillean plateau. 



GEOLOGICAL BASEMENT OF JAMAICA. 



The stratigraphy and older geology of Jamaica have been partially 

 described by the former geologists of the island,* so that only a few of 

 the characteristics which have produced effects upon the later geomorphy 

 need be considered. 



The nucleus of the island, as may be seen in the higher mountains, 

 consists of some metamorphic shales and igneous rocks of a syenitic, 

 dioritic, and porphyritic nature which are usually decayed and covered 

 with very deep soil. Round these accumulations, compact Cretaceous 

 limestones succeeded by Cretaceous sands and marls occur in narrow 

 belts. The igneous rocks are however younger than the Cretaceous 

 series, for dykes of them cut into this formation. The easy atmospheric 

 degradation of these rocks covered with loose soil suggests the com- 

 paratively recent great elevation of the higher mountains, thus favour- 

 ing the uncovering of the older formations. These formations are next 

 succeeded by a great thickness of what the Jamaican geologists called 

 " Lower Tertiary or Conglomerate Series," consisting of sandstones, 

 grits, shales and massive conglomerates. Whether this series be 

 entirely Tertiary, or partly older, is immaterial in this study, for we 

 shall not attempt to reconstruct the geography of that date, or to 

 determine the sources whence the fragments of the crystalline rocks and 

 mechanical deposits of the island came. 



*Geology of Jamaica, pp. 1-339, i86g. By James Sawkins, J. P. Wall, Lucas Barrett, Arthur Lennox, C. 

 B. Brown, and Robert Etheridge. 



