112 bulletin: museum of comparative zoology. 



of the parish;^ according to Lennox,^ "the granite in the adjacent 

 reo-ions of St. Thomas-in-the-Yale is certainly posterior in date to the 

 White Limestone." 



The numerous observations of the Jamaican geologists, verified by our 

 own, justify the opinion expressed by most of them that these so called 

 granites are of Tertiary age,* and we think at least of a date later 

 than the Montpelier epoch, — a conclusion borne out not only by the 

 facts presented but also by the absence of the debris of these rocks in 

 the conglomerates so abundant in the preceding formations. 



From the fact that the composition of these rocks shows them all to 

 be of deep seated origin, and from their occurrence as masses beneath 

 various formations into which they send dikes and sills, and owing to 

 the fact that no evidence exists that they were ever protruded to the 

 surface, we believe they represent the various phenomena of a great lac- 

 colith which in Tertiary (Middle Oligocene) time was protruded upward 

 into the then existing formations of the Jamaican sequence. Further- 

 more, it is our opinion that the intrusion of these rocks was associated 

 with the elevation of the island, and the progressive shallowing so evi- 

 dent in the ascending sequence of the White Limestones, whereby they 

 changed from the deep sea foraminiferal deposits of the Montpelier beds 

 into more shallow limestones, and, finally, into the land areas which existed 

 at the time of the peripheral deposition of the Bowden Oligocene. 



IIL The Low Layton Eruptives. — At only one locality on the island 

 is there supposed evidence of the occurrence of true eruptive rocks in 

 situ, or of igneous rocks of later age than those already described. This 

 is at the Black Hill near Low Layton, adjacent to Savanna Point, on 

 the north coast, between Bluff Bay and Hope Bay, in the parish of Port- 

 land. This is a ridge extending from the seacoast near Retreat to near 

 Low Layton, accompanied by one or two outliers and reaching an alti- 

 tude of 700 feet. Here it has been asserted there is a neck or stock, 

 or possibly the remains of old lava flows of what was once apparently a 

 true volcano protruding through the Pliocene limestones. This hill has 

 undergone great denudation. 



1 Jamaican Reports, p. 146, 



2 Ibid., p. 147. 



' Barrett (in the Jamaican Reports, p. 81) states: "The porphyry has altered 

 the Cretaceous and Eocene rocks, and it is contemporaneous with the lower part of 

 tlie White limestone scries, so that it was erupted after the deposition of the Eocene 

 (Black Shale) series, but before tlie formation of the Pliocene (White limestone?) 

 strata; its geological age is therefore Miocene" (Oligocene). See also pages 146, 

 182, of same report for like opinions of other writers. 



