Geological Society. 135 
laceous substance, among which strings of chalcedony are some- 
times found. It is remarkable, that the only instance of a 
similar structure which has occurred to the author, is in an 
amygdaloidal rock, decidedly of volcanic origin, at Black Hill, - 
on another part of the island. 
. These trap rocks are found, generally, supporting the great 
white limestone formation, which occupies a very large portion 
of the whole island. This formation, from the fossils it con- 
tains, is referred by Mr. De la Beche to the tertiary series. It 
is principally composed of white limestone, most frequently 
very compact, and then strongly resembling the compact va- 
rieties of Jura limestone. The strata are usually very thick, 
varying from 3 to 20 feet in breadth. In some districts, this 
rock is interstratified with thick beds of red marle, and sand- 
stone, and white chalky marle. The compact limestone con- 
stitutes the middle part of the formation: the lower beds con- 
sist, chiefly, of sands and marles, sometimes associated with 
blueish gray compact limestones, at others with beds of earthy 
yellowish white limestone, containing an abundance of organic 
remains, viz. Eichinites, Ostree, and a particularly large species 
of Cerithium. 'The upper beds of the formation are rather 
chalky, sandy, and marly, and contain numerous remains of 
the genera Conus, Cerithium, Astarte, Natica, &c.; and near 
the sea coast a great quantity of corals, which, frequently, have 
almost a recent appearance. 
Above the white limestone formation, beds of conglomerate 
and sandstone are visible on many points, particularly on the 
edges of the savannahs; whence the author calls them the 
Savannah sandstones. 
The upper beds of all visible in the island, consist of Dz- 
luvium and Alluvium. ‘The former shows itself on a very 
large scale, covering the surface of the principal plains, par- 
ticularly that of Liguanea. It consists of rounded fragments 
of the rocks which compose the neighbouring mountains. The 
Hope river, which has cut its channel through the plain of 
Liguanea, has exposed sections of these diluvial gravel-beds, 
from 200 to 300 feet in thickness. The greater part of the 
large plain of Vere and Clarendon is also composed of dilu- 
vium. The pebbles of these beds consist chiefly of trap rocks ; 
those of white limestone are comparatively rare, this rock ap- 
pearing to have failed in resistance to the force of attrition by 
which its fragments were attacked. The separation between 
_ the diluvium and alluvium is not very decided ; but deposits 
’ of the latter class have certainly been produced, in consider- 
able quantities, along the course of many of the rivers; and 
on parts of the shore, particularly between Kingston and Port 
Henderson, 
