Mr. De la Beche on the Geology of Jamaica. 165 



Hope Valley covers the red sandstones and conglomerates, and is here and 

 there mixed with greenstone. Apparently resting upon this, and on the low- 

 est part of the descent, a considerable mass of syenite comes in, very closely 

 resembling that found among the submedial country between Sheldon and 

 Abbey Green, though here associated with more recent rocks : this syenite 

 plunges beneath the inclined diluvial plain of Liguanea, which abuts against 

 it, a few occasional patches of white limestone only intervening. 



The Agua Alta presents a good section of these rocks between the northern 

 base of Stony Hill and Scot's Hall (Maroon Town) : near the former place 

 brownish red porphyritic conglomerate occurs beneath the lower marls of the 

 white limestone formation, and is succeeded by porphyries and other trap 

 rocks of various composition and structure ; the whole of which may be con- 

 sidered as one great mass with sometimes one ingredient predominating and 

 sometimes another. 



It should here be remarked that the trap of Jamaica is often very much de- 

 composed to a considerable depth *, so as not to be always immediately recog- 

 nised ; but by careful examination hard portions will be found forming cliffs, 

 beds of rivers, &c. where the structure can be examined. This decomposition 

 of the Jamaica trap principally takes place in those varieties in which felspar 

 forms a considerable portion of the mass ; it would therefore seem to arise 

 from the decomposition of this mineral, which has given way before the effects 

 of a tropical climate. 



A mountain of syenite composed of greenish black hornblende, flesh-co- 

 loured felspar, and colourless quartz, the latter mineral being rare, rises above 

 Golden Spring estate ( St. Andrew's) on the Agua Alta : this syenite resem- 

 bles that of the descent from St. George's Gap, and is most probably a con- 

 tinuation of it. To enumerate all the varieties of trap found on the Agua 

 Alta would be tedious ; it will be sufficient to state that porphyries, greenstones, 

 and syenites seem to pass into each other. One rock however which occurs 

 about two miles south from Scot's Hall (Maroon Town) deserves attention, being 

 wholly, or very nearly so, composed of two varieties of felspar, differing in 

 colour, so as to have the appearance of syenite at a short distance. This rock 

 would seem referrible to that variety of compact felspar which Dr. MacCulloch 

 denominates "crystalline-granular f:" the felspar however of the rock in ques- 

 tion is common, not compact ; but as that author remarks that " it seems doubt- 

 ful whether this variety does not sometimes consist of an aggregation of com- 

 mon instead of compact felspar X," it appears to be a rock of that class. 



* From this circumstance it is often known in the island by the name of Rotten Stone. 

 t Geological Classification of Rocks, p. 496. % Ibid. p. 497. 



