1904-5.| THE GROWTH OF TRINIDAD. 143 
of Naparima has yet been found. I am of opinion therefore, that all that 
has been published, (and that of course chiefly by myself), is liable to large 
correction. Also the true sequence of the rocks at Pointapier has not been 
made out. The Ditrupabed will probably prove to be much later in age 
than the adjacent strata; and the study of its fossils leads me to believe 
it to be miocene and possibly late miocene, somewhere about the equivalent 
of the tertiary beds of Cumana in Venezuela. 
The soils of the northern division of the island are derived from the 
degradation of the ancient rocks of the Parian Range, whose origin and 
formation I have described. The calcareous part is dissolved and carried 
away almost*entirely by the running waters, some of which hold large quan- 
tities of lime, so that the gravelly and sandy detritus occupying the beds 
of valleys consists almost entirely of the siliceous constituents of the rocks. 
This gravelly and sandy detritus, though it affords a receptacle for water 
wherever there exists a barrier of any kind to prevent it from flowing off 
at once, does not, as clays and marls do, enter into intimate union with its 
contained water. Consequently where narrow passages admit of a slow 
escape of the water, the exhaustion of the reservoir down to the point 
at which it is tapped by the apertures of escape, will depend upon the 
proportion between the latter and the volume of water contained in the 
reservoir. 
Before the great subsidence which separated Trinidad from Venezuela 
all the valleys and ravines running southward in the northern range had 
the same ditchlike character throughout that still characterizes the lower 
portions of the Caura and Aronca valleys. But on the occurrences of the 
dislocations already referred to and particularly that one passing along the 
axis of the Gulf of Paria and through the Bocagrande, as shown on the map 
(Fig. 9), the upper portions of those valleys were thrown down below the 
level of the lower portions. The thrown down upper portions became 
filled-up by gravelly alluvium derived from the waste of the surrounding 
rdges. I need hardly say that the subsidence was slow and that the 
basin-shaped expansions were never in the condition of lakes or even of 
swamps. Filling-up was always much in advance of subsidence. Never- 
theless though not actually lakes these basin-shaped cavities became vast 
underground reservoirs of water which escapes slowly at their lower ex- 
tremities. The perennial character of the streams of the northern part of 
the island is in a large measure due to the phenomena described. I will 
endeavour by the aid of diagrams to make the case more clear. The figures 
and descriptions given are not those of any particular valley but of a 
generalized or ideal valley illustrating the peculiar features common to 
most of the valleys under consideration. 
