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This agate, of which amulets and signet-rings were and are still made, and which takes many varieties of tints, lies in veins mostly striking east-west; and varying in thickness from an inch to several feet. The sequence is grey granite below, the band of chalcedony, and above it a curious schistose gneiss-formation.
Northward the granitic chain of the Silla de Caracas and Porto Cabello are separated from the Llanos by a screen of mountains that are schistose between Villa de Cura and Parapara, and calcareous between the Bergantin and Caripe. I was no less struck by this absence of blocks on the banks of the Amazon.
On a point of land at the distance of three quarters of a mile to the W.b.S. of the tents, and within a hundred yards of the sea, the remains of six Esquimaux huts were discovered; they consisted of rude circles, about six feet in diameter, constructed irregularly of stones of all sizes and shapes, and raised to the height of two feet from the ground: they were paved with large slabs of white schistose sandstone, which is here abundant; the moss had spread over this floor, and appeared to be the growth of three or four years.
This stratum continues for sixteen to twenty feet above the water; for the next ten feet there is a horizontal stratum of black schistose rock, which was of so soft a consistence, that the weather had excavated several tiers of galleries, upon the roof and sides of which some curious drawings were observed, which deserve to be particularly described.
They all refer to the repeated alterations, often in undulatory planes, of glassy, pearly, stony, and crystalline layers: the crystalline layers, however, seem to be much more perfectly developed at Ascension, than in the above-named countries. Humboldt compares some of the stony beds, when viewed from a distance, to strata of a schistose sandstone.
Down towards either horn run two ranges of schistose rocks, about 1,500 feet high, their sides almost precipitous, and the topmost ridge as sharp as a knife, and jagged as a saw; the intervening space is entirely filled up by an enormous glacier, which, descending with one continuous incline from the head of a valley on the right, and sweeping like a torrent round the roots of an isolated clump of hills in the centre rolls at last into the sea.
It also passes in the same uninterrupted manner into a basalt, and at length into a soft claystone, with a schistose tendency on exposure, in no respect differing from those of the trap islands of the western coast." The same author mentions, that in Shetland a granite composed of hornblende, mica, feldspar, and quartz graduates in an equally perfect manner into basalt.
That large portions of the Plutonic rocks had been formed under the influence of such intense heat is in perfect accordance with their great volume, uniform composition, and absence of stratification. The forcing also of veins into contiguous stratified or schistose rocks is a natural consequence of the hydrostatic pressure to which columns of molten matter many miles in height must give rise.
On the top of this hill we deposited a bottle containing a short notice of our visit, and raised over it a small mound of stones; of these we found no want, for the surface was covered with small pieces of schistose limestone, and nothing like soil or vegetation could be seen.
These, however, were sufficient to show me that the gneiss of Depilto was overlain conformably by the contorted schists; that the latter were followed by soft trappean beds, and these by thick beds of quartz-conglomerate, apparently derived from the degradation of the schistose rocks, with their numerous quartz veins.
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