ST.JAGO, IN THE CAPE DE VERDE ARCHIPELAGO.
Rocks of the lowest series.
A calcareous sedimentary deposit, with recent shells, altered by the contact of superincumbent lava, its horizontality and extent.
Subsequent volcanic eruptions, associated with calcareous matter in an earthy and fibrous form, and often enclosed within the separate cells of the scoriae.
Ancient and obliterated orifices of eruption of small size.
Difficulty of tracing over a bare plain recent streams of lava.
Inland hills of more ancient volcanic rock.
Decomposed olivine in large masses.
Feldspathic rocks beneath the upper crystalline basaltic strata.
Uniform structure and form of the more ancient volcanic hills.
Form of the valleys near the coast.
Conglomerate now forming on the sea beach.
(FIGURE 1: MAP 1: PART OF ST.JAGO, ONE OF THE CAPE DE VERDE ISLANDS.)The island of St.Jago extends in a N.N.W.and S.S.E.direction, thirty miles in length by about twelve in breadth.My observations, made during two visits, were confined to the southern portion within the distance of a few leagues from Porto Praya.The country, viewed from the sea, presents a varied outline: smooth conical hills of a reddish colour (like Red Hill in Figure 1 (Map 1).(The outline of the coast, the position of the villages, streamlets, and of most of the hills in this woodcut, are copied from the chart made on board H.M.S."Leven." The square-topped hills (A, B, C, etc.)are put in merely by eye, to illustrate my description.)), and others less regular, flat-topped, and of a blackish colour (like A, B, C,) rise from successive, step-formed plains of lava.At a distance, a chain of mountains, many thousand feet in height, traverses the interior of the island.There is no active volcano in St.Jago, and only one in the group, namely at Fogo.The island since being inhabited has not suffered from destructive earthquakes.
The lowest rocks exposed on the coast near Porto Praya, are highly crystalline and compact; they appear to be of ancient, submarine, volcanic origin; they are unconformably covered by a thin, irregular, calcareous deposit, abounding with shells of a late tertiary period; and this again is capped by a wide sheet of basaltic lava, which has flowed in successive streams from the interior of the island, between the square-topped hills marked A, B, C, etc.Still more recent streams of lava have been erupted from the scattered cones, such as Red and Signal Post Hills.The upper strata of the square-topped hills are intimately related in mineralogical composition, and in other respects, with the lowest series of the coast-rocks, with which they seem to be continuous.
MINERALOGICAL DESCRIPTION OF THE ROCKS OF THE LOWEST SERIES.
These rocks possess an extremely varying character; they consist of black, brown, and grey, compact, basaltic bases, with numerous crystals of augite, hornblende, olivine, mica, and sometimes glassy feldspar.A common variety is almost entirely composed of crystals of augite with olivine.Mica, it is known, seldom occurs where augite abounds; nor probably does the present case offer a real exception, for the mica (at least in my best characterised specimen, in which one nodule of this mineral is nearly half an inch in length) is as perfectly rounded as a pebble in a conglomerate, and evidently has not been crystallised in the base, in which it is now enclosed, but has proceeded from the fusion of some pre-existing rock.
These compact lavas alternate with tuffs, amygdaloids, and wacke, and in some places with coarse conglomerate.Some of the argillaceous wackes are of a dark green colour, others, pale yellowish-green, and others nearly white; I was surprised to find that some of the latter varieties, even where whitest, fused into a jet black enamel, whilst some of the green varieties afforded only a pale gray bead.Numerous dikes, consisting chiefly of highly compact augitic rocks, and of gray amygdaloidal varieties, intersect the strata, which have in several places been dislocated with considerable violence, and thrown into highly inclined positions.One line of disturbance crosses the northern end of Quail Island (an islet in the Bay of Porto Praya), and can be followed to the mainland.
These disturbances took place before the deposition of the recent sedimentary bed; and the surface, also, had previously been denuded to a great extent, as is shown by many truncated dikes.
DESCRIPTION OF THE CALCAREOUS DEPOSIT OVERLYING THE FOREGOING VOLCANICROCKS.
This stratum is very conspicuous from its white colour, and from the extreme regularity with which it ranges in a horizontal line for some miles along the coast.Its average height above the sea, measured from the upper line of junction with the superincumbent basaltic lava, is about sixty feet; and its thickness, although varying much from the inequalities of the underlying formation, may be estimated at about twenty feet.It consists of quite white calcareous matter, partly composed of organic debris, and partly of a substance which may be aptly compared in appearance with mortar.Fragments of rock and pebbles are scattered throughout this bed, often forming, especially in the lower part, a conglomerate.Many of the fragments of rock are whitewashed with a thin coating of calcareous matter.
At Quail Island, the calcareous deposit is replaced in its lowest part by a soft, brown, earthy tuff, full of Turritellae; this is covered by a bed of pebbles, passing into sandstone, and mixed with fragments of echini, claws of crabs, and shells; the oyster-shells still adhering to the rock on which they grew.Numerous white balls appearing like pisolitic concretions, from the size of a walnut to that of an apple, are embedded in this deposit;they usually have a small pebble in their centres.Although so like concretions, a close examination convinced me that they were Nulliporae, retaining their proper forms, but with their surfaces slightly abraded:
