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



From the above it appears that the grhs rouge of M. Humboldt forms a 

 considerable part of that portion of the American continent which was visited 

 by him, and that it there constitutes a formation of very considerable thick- 

 ness : it is therefore by no means surprising that we should find it in Jamaica 

 also of considerable thickness, (Valley of the Hope River, &c.) and it is by 

 no means improbable that it may also be found in the islands of Cuba and 

 Hayti. 



The Mexican red sandstone is stated to present the most striking resem- 

 blance to the Rathe todte liegende of Mansfield, to contain angular pieces of 

 Lydian stone, syenite, porphyry, quartz, and hornstone. The cement uniting 

 these fragments is argillo-ferruginous, very tenacious, yellowish brown and 

 often brick red. 



This Venezuela gres rouge is represented to be a conglomerate, containing 

 rounded fragments of quartz, Lydian stone, and flinty slate, united by an ar- 

 gillo-ferruginous cement, of an olive brown colour, and extremely tenacious. 

 The large-grained conglomerate alternates with a very fine-grained sand- 

 stone (Mesa de Paja). 



The New Grenada rock is composed of alternating beds of small-grained 

 quartzose and schistose sandstone, with conglomerates containing angular 

 fragments (from 2 to 3 inches in breadth) of Lydian stone, clay-slate, gneiss, 

 and quartz (Honda, Espinal). The cement is argillaceous and ferruginous, 

 sometimes siliceous, and the colours of the rock vary from yellow gray to 

 brownish red. 



The gres rouge of the Quito plateau is stated to be coloured by iron, it is 

 generally very argillaceous, with small grains of quartz ; it is sometimes 

 schistose, and alternates, as in Thuringia, with a conglomerate containing 

 fragments of porphyry, three, five, and even nine inches in diameter. Clay 

 beds are found in this formation, sometimes brown, at others white and stea- 

 titic, passing into the claystones of the red sandstone porphyries. 



From this description of the American gres rouge, it will be found to cor- 

 respond, as nearly as could, from the distances, be expected, with the red 

 sandstones and conglomerates, coal measures and porphyritic conglomerates 

 of Jamaica. Porphyritic pebbles are it would appear more abundant in the 

 Jamaica conglomerates than in those of the opposite continent: this is how- 

 ever a difference of no great importance, as the fragments found in this class 

 of rocks are generally derived from the older rocks of the districts in which 

 they occur. 



White limestone formation. — This formation is much more difficult to iden- 

 tify with M. Humboldt's description of American rocks than the preceding. 



