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HORNBLENDE-SCHIST is usually black, and composed principally of hornblende, with a variable quantity of feldspar, and sometimes grains of quartz. When the hornblende and feldspar are in nearly equal quantities, and the rock is not slaty, it corresponds in character with the greenstones of the trap family, and has been called "primitive greenstone." It may be termed hornblende rock, or amphibolite.
Nevertheless, these greenstones appear to belong, like the lavas, to the Lower Silurian period. The Lingula beds in North Wales have been described as 5000 feet in thickness. In the upper portion of these deposits volcanic tuffs or ashy materials are interstratified with ordinary muddy sediment, and here and there associated with thick beds of feldspathic lava.
But in many places a variety occurs which is composed simply of feldspar and hornblende; and in examining more minutely this duplicate compound, it is observed in some places to assume a fine grain, and at length to become undistinguishable from the greenstones of the trap family.
Sedimentary strata have been carried down, by movements of the earth's crust, far below the surface, covered by other deposits, and subjected to great heat, which, aided by the water contained in the rocks and various chemical reactions, has effected a re-arrangement of the mineral contents of the strata, so that by molecular movements, the metamorphic crystalline rocks, including interstratified granites and greenstones, have been formed.
Thus on Wollaston Island slate and grauwacke can be distinctly traced passing into feldspathic rocks and greenstones, including iron pyrites and epidote, but still retaining traces of cleavage with the usual strike and dip.
At Jajuel I was informed that the copper-ore, with some gold, is found only in the greenstones and altered feldspathic clay-slate, which alternate with the purple porphyritic conglomerate.
Carried to greater depths and subjected to more intense heat, the strata have been completely fused, and the liquid or pasty mass, invading the contorted strata above it, has formed perfectly crystalline intrusive granites and greenstones.
Even during the few years I was in Nicaragua there were some modifications of the surface effected; I saw the commencement of new valleys, and the widening and lengthening of others, caused not only by the gradual denudation of the surface, but by landslips, some of which occur every wet season. The rocks of the district are dolerytes, with bands and protrusions of hard greenstones.
Relation between movements of subsidence and volcanic action. Pampean formation. Recent elevatory movements. Long-continued volcanic action in the Cordillera. Conclusion. I have already described the general nature of the rocks in the low country north of Valparaiso, consisting of granites, syenites, greenstones, and altered feldspathic clay-slate.
But there are greenstones associated with the same formation, which, although they are often conformable to the slates, are in reality intrusive rocks. They alter the stratified deposits both above and below them, and when traced to great distances are sometimes seen to cut through the slates, and to send off branches.
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