112 BULLETIN: MUSEUM OF COMPARATIVE ZOÖLOGY. 
of the parish;! according to Lennox,? “the granite in the adjacent 
regions of St. Thomas-in-the-Vale 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 débris of these rocks iu 
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. 
III. 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, ot 
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 nea 
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 ® 
true volcano protruding through the Pliocene limestones, This hill has 
undergone great denudation. 
1 Jamaican Reports, p. 146, 
2 Ibid., p. 147. 
3 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 O 
the White limestone series, so that it was erupted after the deposition of the Eocen* 
(Black Shale) series, but before the formation of the Pliocene (White limestone ' 
strata; its geological age is therefore Miocene" (Oligocene). See also pages 140 
182, of same report for like opinions of other writers. 
