Some ancient submarine volcanic rocks are worth mentioning, from their rarity on this eastern side of the continent.In the valley of the Tapas (fifty or sixty miles N.of Maldonado) there is a tract three or four miles in length, composed of various trappean rocks with glassy feldspar--of apparently metamorphosed grit-stones--of purplish amygdaloids with large kernels of carbonate of lime (Near the Pan de Azucar there is some greenish porphyry, in one place amygdaloidal with agate.)--and much of a harshish rock with glassy feldspar intermediate in character between claystone porphyry and trachyte.This latter rock was in one spot remarkable from being full of drusy cavities, lined with quartz crystals, and arranged in planes, dipping at an angle of 50 degrees to the east, and striking parallel to the foliation of an adjoining hill composed of the common mixture of quartz, feldspar, and imperfect hornblende: this fact perhaps indicates that these volcanic rocks have been metamorphosed, and their constituent parts rearranged, at the same time and according to the same laws, with the granitic and metamorphic formations of this whole region.In the valley of the Marmaraya, a few miles south of the Tapas, a band of trappean and amygdaloidal rock is interposed between a hill of granite and an extensive surrounding formation of red conglomerate, which (like that at the foot of the S.Animas) has its basis porphyritic with crystals of feldspar, and which hence has certainly suffered metamorphosis.
MONTE VIDEO.
The rocks here consist of several varieties of gneiss, with the feldspar often yellowish, granular and imperfectly crystallised, alternating with, and passing insensibly into, beds, from a few yards to nearly a mile in thickness, of fine or coarse grained, dark-green hornblendic slate; this again often passing into chloritic schist.These passages seem chiefly due to changes in the mica, and its replacement by other minerals.At Rat Island I examined a mass of chloritic schist, only a few yards square, irregularly surrounded on all sides by the gneiss, and intricately penetrated by many curvilinear veins of quartz, which gradually BLEND into the gneiss: the cleavage of the chloritic schist and the foliation of the gneiss were exactly parallel.Eastward of the city there is much fine-grained, dark-coloured gneiss, almost assuming the character of hornblende-slate, which alternates in thin laminae with laminae of quartz, the whole mass being transversely intersected by numerous large veins of quartz: Iparticularly observed that these veins were absolutely continuous with the alternating laminae of quartz.In this case and at Rat Island, the passage of the gneiss into imperfect hornblendic or into chloritic slate, seemed to be connected with the segregation of the veins of quartz.(Mr.Greenough page 78 "Critical Examination" etc., observes that quartz in mica-slate sometimes appears in beds and sometimes in veins.Von Buch also in his "Travels in Norway" page 236, remarks on alternating laminae of quartz and hornblende-slate replacing mica-schist.)The Mount, a hill believed to be 450 feet in height, from which the place takes its name, is much the highest land in this neighbourhood: it consists of hornblendic slate, which (except on the eastern and disturbed base) has an east and west nearly vertical cleavage; the longer axis of the hill also ranges in this same line.Near the summit the hornblende-slate gradually becomes more and more coarsely crystallised, and less plainly laminated, until it passes into a heavy, sonorous greenstone, with a slaty conchoidal fracture; the laminae on the north and south sides near the summit dip inwards, as if this upper part had expanded or bulged outwards.This greenstone must, I conceive, be considered as metamorphosed hornblende-slate.The Cerrito, the next highest, but much less elevated point, is almost similarly composed.In the more western parts of the province, besides gneiss, there is quartz-rock, syenite, and granite; and at Colla, Iheard of marble.
Near M.Video, the space which I more accurately examined was about fifteen miles in an east and west line, and here I found the foliation of the gneiss and the cleavage of the slates generally well developed, and extending parallel to the alternating strata composed of the gneiss, hornblendic and chloritic schists.These planes of division all range within one point of east and west, frequently east by south and west by north; their dip is generally almost vertical, and scarcely anywhere under 45 degrees: this fact, considering how slightly undulatory the surface of the country is, deserves attention.Westward of M.Video, towards the Uruguay, wherever the gneiss is exposed, the highly inclined folia are seen striking in the same direction; I must except one spot where the strike was N.W.by W.The little Sierra de S.Juan, formed of gneiss and laminated quartz, must also be excepted, for it ranges between [N.to N.E.] and [S.
to S.W.] and seems to belong to the same system with the hills in the Maldonado district.Finally, we have seen that, for many miles northward of Maldonado and for twenty-five miles westward of it, as far as the S.de las Animas, the foliation, cleavage, so-called stratification and lines of hills, all range N.N.E.and S.S.W., which is nearly coincident with the adjoining coast of the Atlantic.Westward of the S.de las Animas, as far as even the Uruguay, the foliation, cleavage, and stratification (but not lines of hills, for there are no defined ones) all range about E.by S.and W.by N., which is nearly coincident with the direction of the northern shore of the Plata; in the confused country near Las Minas, where these two great systems appear to intersect each other, the cleavage, foliation, and stratification run in various directions, but generally coincide with the line of each separate hill.
SOUTHERN LA PLATA.