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 debris 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. AVall has noted from the Port 

 Maria bluff (see Plate XIIL), 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." ^ 



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 a 



1 Jamaican Reports, p. 130. 



