第99章 LUTONIC AND METAMORPHIC ROCKS:--CLEAVAGE AND FOLIA

The cleavage of the homogeneous schists, the foliation of those composed of more or less distinct minerals in layers, and the planes of alternation of the different varieties or so-called stratification, are all parallel, and preserve over this 200 miles of coast a remarkable degree of uniformity in direction.At the northern end of the group, at Low's Harbour, the well-defined folia of mica-schist everywhere ranged within eight degrees (or less than one point of the compass) of N.19 degrees W.and S.19 degrees E.; and even the point of dip varied very little, being always directed to the west and generally at an angle of forty degrees; I should mention that I had here good opportunities of observation, for I followed the naked rock on the beach, transversely to the strike, for a distance of four miles and a half, and all the way attended to the dip.Along the outer islands for 100 miles south of Low's Harbour, Lieutenant Stokes, during his boat-survey, kindly observed for me the strike of the foliation, and he assures me that it was invariably northerly, and the dip with one single exception to the west.Further south at Vallenar Bay, the strike was almost universally N.25 degrees W.and the dip, generally at an angle of about 40degrees to W.25 degrees S., but in some places almost vertical.Still farther south, in the neighbourhood of the harbours of Anna Pink, S.

Estevan and S.Andres, and (judging from a distance) along the southern part of Tres Montes, the foliation and cleavage extended in a line between [N.11 degrees to 22 degrees W.] and [S.11 degrees to 22 degrees E.]; and the planes dipped generally westerly, but often easterly, at angles varying from a gentle inclination to vertical.At A.Pink's Harbour, where the schists generally dipped easterly, wherever the angle became very high, the strike changed from N.11 degrees W.to even as much as N.45 degrees W.:

in an analogous manner at Vallenar Bay, where the dip was westerly (viz.on an average directed to W.25 degrees S.), as soon as the angle became very high, the planes struck in a line more than 25 degrees west of north.The average result from all the observations on this 200 miles of coast, is a strike of N.19 degrees W.and S.19 degrees E.: considering that in each specified place my examination extended over an area of several miles, and that Lieutenant Stokes' observations apply to a length of 100 miles, Ithink this remarkable uniformity is pretty well established.The prevalence, throughout the northern half of this line of coast, of a dip in one direction, that is to the west, instead of being sometimes west and sometimes east, is, judging from what I have elsewhere seen, an unusual circumstance.In Brazil, La Plata, the Falkland Islands, and Tierra del Fuego, there is generally an obvious relation between the axis of elevation, the outline of the coast, and the strike of the cleavage or foliation: in the Chonos Archipelago, however, neither the minor details of the coast-line, nor the chain of the Cordillera, nor the subordinate transverse mountain-axes, accord with the strike of the foliation and cleavage: the seaward face of the numerous islands composing this Archipelago, and apparently the line of the Cordillera, range N.11 degrees E., whereas, as we have just seen, the average strike of the foliation is N.19 degrees W.

There is one interesting exception to the uniformity in the strike of the foliation.At the northern point of Tres Montes (latitude 45 degrees 52minutes) a bold chain of granite, between two and three thousand feet in height, runs from the coast far into the interior, in an E.S.E.line, or more strictly E.28 degrees S.and W.28 degrees N.(In the distance, other mountains could be seen apparently ranging N.N.E.and S.S.W.at right angles to this one.I may add, that not far from Vallenar Bay there is a fine range, apparently of granite, which has burst through the mica-slate in a N.E.by E.and S.W.by S.line.) In a bay, at the northern foot of this range, there are a few islets of mica-slate, with the folia in some parts horizontal, but mostly inclined at an average angle of 20 degrees to the north.On the northern steep flank of the range, there are a few patches (some quite isolated, and not larger than half a-crown!) of the mica-schist, foliated with the same northerly dip.On the broad summit, as far as the southern crest, there is much mica-slate, in some places even 400 feet in thickness, with the folia all dipping north, at angles varying from 5 degrees to 20 degrees, but sometimes mounting up to 30 degrees.The southern flank consists of bare granite.The mica-slate is penetrated by small veins of granite, branching from the main body.(The granite within these veins, as well as generally at the junction with the mica-slate, is more quartzose than elsewhere.The granite, I may add, is traversed by dikes running for a very great length in the line of the mountains; they are composed of a somewhat laminated eurite, containing crystals of feldspar, hornblende, and octagons of quartz.) Leaving out of view the prevalent strike of the folia in other parts of this Archipelago, it might have been expected that they would have dipped N.28 degrees E., that is directly from the ridge, and, considering its abruptness, at a high inclination; but the real dip, as we have just seen, both at the foot and on the northern flank, and over the entire summit, is at a small angle, and directed nearly due north.From these considerations it occurred to me, that perhaps we here had the novel and curious case of already inclined laminae obliquely tilted at a subsequent period by the granitic axis.Mr.