Vol. 58.] PHYSICAL DEVELOPMENT OF DOMINICA, ETC. B47 
between 100 and 200 feet. Grenada, about the size of St. Vincent, 
with one dome rising to 2749 feet, owes its generally greater 
elevation above the sea to more recent volcanic accumulations. 
LV. Tue Votcantc Formations oF Dominica. 
The Basement. 
In Antigua, a basement of igneous rocks underlies tufaceous. 
deposits, in the upper portions of which are calcareous layers 
containing fossils belonging to the Oligocene or to the Eocene 
formations. These early Tertiary beds are succeeded only by late 
Pliocene or more recent deposits. In Dominica there is the same 
foundation of old igneous rocks, which, however, are not succeeded 
by calcareous beds, but by massive tuffs. These are overlain by 
Pleistocene or recent volcanic accumulations, which form the high 
ridges and domes. This contiguous occurrence of older and newer 
volcanic formations has popularly given rise to confusion. It is not 
easy to decipher the physical features of these islands, except 
by going beyond their mountainous portions into their calcareous 
zones, which are almost wanting in Dominica. 
The sea has made encroachments upon the shore, south of Roseau 
and at Point Michel, 4 miles distant ; the foundations of the cliffs 
are seen to consist of a dark-grey doleritic rock, much jointed and 
fractured, and overlain by a firmly-cemented volcanic breccia or 
conglomerate. The advantage of an examination at this point is 
that the rocks are worn away more rapidly by the waves than by 
decay, thus giving a clean section. But in the exposures of the 
interior of the island (observed to an elevation of about 1500 feet), 
where all the later accumulations have been removed, the older vol- 
canic rocks are found to be decomposed and converted into ared clay 
to depths of 8 feet or more, in places disclosing residual boulders of 
decomposition, such as those on the road to Bona Vista, and eastward 
of the Belleville divide leading to Geneva. On account of their funda- 
mental position and their long exposure (shown by the decayed 
surfaces), as well as by their similarity to the older rocks of 
Guadeloupe, which pass under Tertiary formations, it is probable 
that these rocks belong toa pre-Tertiary period. As before noticed, 
this basement-formation is succeeded by a very much disturbed 
accumulation of breccia. 
In the higher valleys, an old volcanic tuff recurs, as seen to an 
altitude of 1300 feet on the road to the Belleville divide. The beds 
are stratified, the individual members being as much as 10 feet 
thick. In places they enclose coarse fragmental materials, and at 
times there are difficulties in distinguishing them from a newer tuff- 
deposit, but they are more widely distributed and their surfaces have 
been subjected to a greater amount of denudation. Their age cannot 
be conjectured with any approach to certainty: they may be the 
equivalent of the old Tertiary tuffs underlying the calcareous beds 
of Antigua and Guadeloupe, or perhaps they are the equivalent of 
some of the Tertiary calcareous beds of those islands. 
