104 PROCEEDINGS OF THE SCIENTIFIC ASSOCIATION. 
In Trinidad the northern range of mountains is composed 
of rocks, belonging chiefly if not entirely to the “ Caribean 
Group.” These rocks attain collectively a thickness of more 
than 10,000 feet. The whole thickness of the group is 
probably much greater than this ; for the evidence furnished 
by dip and other circumstances leads to the inference that a 
portion of the series, as developed in Venezuela, is inferior 
an position to any of the rocks exposed in Trinidad. In the 
diagram, Fig. 3, this older portion is represented as thrown 
down to the north of Trinidad, beneath the waters of the 
Caribean Sea (Fig. 3, a). Besides this downthrow we have 
evidence of two or three other lines of dislocation which 
traverse the range through its length from east to west. 
These dislocations are marked with a star in Fig. 3, and are 
indicated on the sketch map, Fig. 1. They have caused, in 
conjunction with other movements, of which I shall presently 
speak, some peculiar phenomena in the physical geography 
of the valleys, which are much narrower, and in some cases 
quite ditch-like, to the south of the line of the greatest dis- 
location (between c and d, in Fig. 3), and widen out above 
into large basins. 
The separation of Trinidad from Venezuela was probably 
produced by a great downthrow, which I have attempted to 
represent in Fig. 2. The line of that downthrow, passing 
through the Boca Grande, is laid down in the sketch map, 
Fig. 1. From the facts intended to be illustrated by these 
diagrams, it would appear that the Gulf of Paria occupies an 
area of depression, the lowest axis of which passes through 
the Boca Grande, running approximately north and south. 
The amount of subsidence diminishes gradually as we pass 
eastward, until at the valley of Arouca its effects disappear 
