hill: geology of JAMAICA. 113 



This material and the accompanying phenomena of the outcrop have 

 been described by Brown, ^ as follows : — 



" The kinds of rock of which it is composed are amygdaloid and a 

 pasty-looking brown agglomerate. The amygdaloid is composed of a 

 hard blackish base or matrix containing kernels of carbonate of lime, 

 and the agglomerate is made up of blocks of this amygdaloid embedded 

 in a reddish iron stained material, which is extremely siliceous. Down 

 the slope from the top of the volcanic ridge to the valley the bare patches 

 of brownish lava, with blocks of amygdaloid sticking in them, evidently 

 indicate the direction of the old lava flows. Masses of rock jut out from 

 the face of the hill, which have all the appearance of huge rubble walls 

 cemented witli lime; on examination they are found to consist of large 

 blocks of lava intermingled with white limestone. In these the lime- 

 stone has become cherty." 



Two hand specimens of this rock petrographically studied by Cross 

 were determined by him as follows : — 



No. 143. — District of St. George. Low Layton. Basalt. (Plagio- 

 clastic.) Fresh typical basalt, holocrystalline, vesicles filled with crys- 

 talline calcite. No Globigerina. 



No. 141. — liOw Layton. Dense reddish groundmass obscured by fer- 

 ritic material and containing olivine (altered), plagioclase, and augite. 

 Vesicles partly filled with very distinct Globigerina. 



Since the date of Brown's observations, the Jamaican Railway has tun- 

 nelled through the base of the hill, but the only new data it contributes 

 is the fact that the igneous rocks are encountered 700 feet below the 

 summit of the hill, and shows the latter to be of the nature of a volcanic 

 neck rather than an entirely superficial lava flow. 



It is also probably older than the Post-Pliocene to which Brown as- 

 signed it. Our reasons for the latter conclusions are as follows. The 

 fact that the debris of the basalt is intermingled with the flint bearing 

 white limestone of Vicksburg age (supposed to be Miocene or later by the 

 Jamaican Geological Survey) does not necessarily prove the Post-Eocene 

 age of the basalt. On the contrary, the presence of the characteristic 

 fossils and chalk of the Montpelier limestone in the scoriaceous cavities 

 of the basalt clearly shows that this limestone was deposited contem- 

 poraneously with or after the basalt. The yellow calcareous marls de- 

 scribed as lying almost horizontally upon portions of the volcano were 

 tentatively considered at the time of our examination to be the Buff 



1 Jamaican Reports, p. 120. These rocks were also mentioned by De la Beche, 

 op. cit., pp 185-187. 



VOL. XXXIV. 8 



