Page:Quarterly Journal of the Geological Society of London, vol. 25.djvu/459

1869.] TATE—GUYANA, IN VENEZUELA. 349 series skirting the Orinoco, and abutting against the northern escarpment of the Itacama range of mountains is indubitably an extension of the newer Parian of Mr. Wall, which occupies the grassy plains to the north of the river. In the neighbourhood of Bolivar its rocks rest against the metamorphic boss on which the city stands, and consist of a yellow sand-rock below, passing upwards into a red sandstone, which in places is highly ferruginous; surmounting the latter is a quartose conglomerate in an iron-stone paste; the pebbles are of the size of marbles and are well water-worn. The whole of this series stretches far away in every direction from Bolivar, and has a slight dip to the north. The broad valley of San Rafael with its low mural cliffs of from thirty to fifty feet in height is excavated in the yellow sand-rock; and the same series of strata forms the banks of the Orinoco. No fossils were observed in the sandstone beds; but elongated cavities within the sand-rock appear to have been occupied by fragments of the stems or roots of plants.

That these arenaceous strata have originated from the wear and tear of the metamorphic rocks is satisfactorily proved by sections exhibiting the relation between the two series (see fig. 2).

Resting on the granitoid gneiss is a loose ferruginous conglomerate containing large blocks of angular quartz, fragments of micaslate, &c., the constituents becoming finer and rounder as they are traced from the insular mass around which they have been collected.

Alluding to the probable origin of the Pitch lake, of Trinidad, Sir Charles Lyell*, quoting Dr. Nugent, writes that "the Orinoco has for ages been rolling down great quantities of woody and vegetable bodies into the surrounding sea, where, by the influence of currents and eddies they may be arrested and accumulated in particular places . . . ., and that these vegetable substances may have undergone those transformations and chemical changes which produce petroleum." My observations do not confirm the speculation put forth by this author as to the part played by the Orinoco. The identity of the arenaceous strata of the Llanos of the Orinoco, which in places are also lignitiferous and asphaltitic, with the similar strata of the Moruga and Caroni series in Trinidad may be considered certain; and as the bed of the Orinoco, from Ciudad Bolivar (certainly so far west) to the apex of the delta, is excavated in these same strata, it is quite clear that the present river-valley is of more recent origin than those beds to the accumulation of which, in the paragraph quoted, it is implied the river has contributed.

From the absence of fossils, other than vegetable remains, in the Llanos-sandstones, and the presence of marine shells at Cumana and in Trinidad, it appears highly probable that the arenaceous series of the Llanos have been deposited in a shallow estuary, whilst the present littoral areas of the same series have been accumulated under marine conditions.

VI. SUPERFICIAL ACCUMULATIONS OF LIMONITE.—A striking feature of the surface of much of the country passed under review, is the presence of a hydrous oxide of iron—the moco de hierro of the


 * Principles of Geology, 9th edit. p. 250.

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