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



porphyry^ and are almost always of some kind of trap rock. The cementing 

 matter is most frequently a reddish brown argillaceous substance of various 

 degrees of hardness ; and the cement is in some places so obscure, that the peb- 

 bles have the appearance of being joined together by a trappean substance. 



I refer these rocks to the same epoch of formation as the porphyritic con- 

 glomerates of Exeter and Teign mouth, which are by many considered as the 

 English equivalent to the German rothe todte liegende, or that part of it 

 which rests upon the coal measures. The rocks of this geological epoch are 

 often, as is well known, found associated with porphyry, which has obtained 

 the name of Porphi/re secondaire with the French, and Porphyr gehirge with 

 the Germans. In like manner the porphyritic conglomerate of Jamaica is so 

 much mixed with porphyries and other trap rocks, that it will be necessary to 

 describe them together. 



The rocks cover what I have considered as coal measures nearly opposite 

 the Botanical Garden on the Hope River, and continue from thence to where 

 the valley opens upon the Liguanea plain near the Hope Tavern. Most of 

 the conglomerate beds are here of a reddish brown colour, and are composed 

 of rounded pieces of porphyries (principally claystone), of various greenstones, 

 and other trap rocks, cemented by an argillaceous substance, which in some 

 cases passes into a kind of trappean claystone : the general dip of the beds is 

 at about an angle of 45° to the S.W., and their thickness varies from two to 

 ten feet. These strata are associated in an obscure manner with porphyry. In 

 one place the latter rock, with a dark claystone base containing crystals of 

 white felspar, has the appearance of a bed interstratified with the conglome- 

 rate ; but this may, as Dr. MacCulloch has shown, easily occur without the 

 porphyry or trap actually forming a bed. 



Not far from the Hope Tavern there is a porphyry, with a reddish brown 

 trappean base containing white crystals of felspar, and higher up the valley 

 there is another with a compact felspathic base containing crystals of white 

 felspar and a few small grains of quartz. 



By reference to the accompanying map, it will be seen that trap and por- 

 phyritic conglomerate extend from that mentioned on the Hope River north- 

 vvesterly into St. Mary's, and south-easterly through Port Royal parish to the 

 Yallahs River in St. David's. I shall in the first place enumerate the rocks 

 and the changes they undergo in this range of country, which for some di- 

 stance separates the red sandstones and conglomerates from the white lime- 

 stone formation, occurring beneath the latter and above the former. 



On the lower part of the descent from St. George's Gap to the plain of 

 Liguanea brownish red porphyritic conglomerate agreeing with that of the 



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