a 
108 BULLETIN: MUSEUM OF COMPARATIVE. ZOOLOGY, 
exact mode of origin is questionable. From its vast quantities, — prob- 
ably aggregating 3,000 feet in thickness, — its eruptive nature, its size 
and freedom from foreign material, and the occasional colonies of Cre- 
taceous species which found temporary foothold during its deposition, 
there can be no doubt but it represents the piled up débris of a great 
volcanic vent or vents which rose from the waters of the Caribbean Sea 
in Cretaceous time, approximately where the island of Jamaica now 
stands, and constituted a nucleal land around which all subsequent ter- 
rigenous formations now entering into the structure of the island, and 
largely made up of its worked over materials, were derived. This fact is 
one of the most apparent in Jamaican history. 
There is some reason to believe that the eruption of the hornblende- 
andesites continued throughout the epoch recorded in the deposition of 
the lower part of the Blue Mountain Series up to the time of the com- 
mencement of the Richmond formation. The thick beds of tuff alter- 
nating with beds of black shale in the Minho beds of the Clarendon 
section are the last of the undoubtedly volcanic formations in the 
series, 
The more regular stratification and composition of the Richmond 
beds, although containing vast amounts of water worn volcanic pebbles, 
indicates that the sedimentary conditions had become more stable and 
were free from the disturbing effects of volcanic outbreaks. The pebble 
and conglomerate beds in the Richmond Series are much water worn 
and distinctly stratified, and occur interbedded with the shales and 
sandstones, or, when traced out, pass continuously into the latter, which 
is largely made up of water-rolled grains of igneous rock, These con- 
glomerates are mostly composed of the hornblende-andesite with rolled 
fragments of Cretaceous limestone. Wall has noted from the Port 
Maria bluff (see Plate XIIT.), from which he collected the Eocene corals 
described by Duncan, a singular admixture of pebbles composed of 
* gneiss and crystalline slates, rocks of which no trace either in situ 
or otherwise had hitherto been noticed in the island.” 4 
In addition to the hornblende-andesites in the Richmond gravel at 
Port Maria, Cross found one which may be a dacite. He describes this 
as follows :— 
“No, 52 a.—Port Maria. (Pebble in Richmond formation.) A prob- 
able surface volcanic rock. It contains many fresh plagioclase crystals 
with glass inclusions, a few augite prisms, and very little magnetite 
The predominant groundmass is cryptocrystalline and may contain 9 
1 Jamaican Reporte, p. 130. 
