Hopkins, so well known from his mathematical investigations, has most kindly calculated the problem: the proposition sent was,--Take a district composed of laminae, dipping at an angle of 40 degrees to W.19 degrees S., and let an axis of elevation traverse it in an E.28 degrees S.line, what will the position of the laminae be on the northern flank after a tilt, we will first suppose, of 45 degrees? Mr.Hopkins informs me, that the angle of the dip will be 28 degrees 31 minutes, and its direction to north 30degrees 33 minutes west.(On the south side of the axis (where, however, Idid not see any mica-slate) the dip of the folia would be at an angle of 77degrees 55 minutes, directed to west 35 degrees 33 minutes south.Hence the two points of dip on the opposite sides of the range, instead of being as in ordinary cases directly opposed to each other at an angle of 180degrees, would here be only 86 degrees 50 minutes apart.) By varying the supposed angle of the tilt, our previously inclined folia can be thrown into any angle between 26 degrees, which is the least possible angle, and 90 degrees; but if a small inclination be thus given to them, their point of dip will depart far from the north, and therefore not accord with the actual position of the folia of mica-schist on our granitic range.Hence it appears very difficult, without varying considerably the elements of the problem, thus to explain the anomalous strike and dip of the foliated mica-schist, especially in those parts, namely, at the base of the range, where the folia are almost horizontal.Mr.Hopkins, however, adds, that great irregularities and lateral thrusts might be expected in every great line of elevation, and that these would account for considerable deviations from the calculated results: considering that the granitic axis, as shown by the veins, has indisputably been injected after the perfect formation of the mica-slate, and considering the uniformity of the strike of the folia throughout the rest of the Archipelago, I cannot but still think that their anomalous position at this one point is someway directly and mechanically related to the intrusion of this W.N.W.and E.S.E.mountain-chain of granite.
Dikes are frequent in the metamorphic schists of the Chonos Islands, and seem feebly to represent that great band of trappean and ancient volcanic rocks on the south-western coast of Tierra del Fuego.At S.Andres Iobserved in the space of half-a-mile, seven broad, parallel dikes, composed of three varieties of trap, running in a N.W.and S.E.line, parallel to the neighbouring mountain-ranges of altered clay-slate; but they must be of long subsequent origin to these mountains; for they intersected the volcanic formation described in the last chapter.North of Tres Montes, Inoticed three dikes differing from each other in composition, one of them having a euritic base including large octagons of quartz; these dikes, as well as several of porphyritic greenstone at Vallenar Bay, extended N.E.
and S.W., nearly at right angles to the foliation of the schists, but in the line of their joints.At Low's Harbour, however, a set of great parallel dikes, one ninety yards and another sixty yards in width, have been guided by the foliation of the mica-schist, and hence are inclined westward at an angle of 45 degrees: these dikes are formed of various porphyritic traps, some of which are remarkable from containing numerous rounded grains of quartz.A porphyritic trap of this latter kind, passed in one of the dikes into a most curious hornstone, perfectly white, with a waxy fracture and pellucid edges, fusible, and containing many grains of quartz and specks of iron pyrites.In the ninety-yard dike several large, apparently now quite isolated, fragments of mica-slate were embedded: but as their foliation was exactly parallel to that of the surrounding solid rock, no doubt these new separate fragments originally formed wedge-shaped depending portions of a continuous vault or crust, once extending over the dike, but since worn down and denuded.
CHILOE, VALDIVIA, CONCEPCION.
In Chiloe, a great formation of mica-schist strikingly resembles that of the Chonos Islands.For a space of eleven miles on the S.E.coast, the folia were very distinct, though slightly convoluted, and ranged within a point of N.N.W.and S.S.E., dipping either E.N.E.or more commonly W.S.W., at an average angle of 22 degrees (in one spot, however, at 60 degrees), and therefore decidedly at a lesser inclination than amongst the Chonos Islands.On the west and north-western shores, the foliation was often obscure, though, where best defined, it ranged within a point of N.by W.
and S.by E., dipping either easterly or westerly, at varying and generally very small angles.Hence, from the southern part of Tres Montes to the northern end of Chiloe, a distance of 300 miles, we have closely allied rocks with their folia striking on an average in the same direction, namely between N.11 degrees and 22 degrees W.Again, at Valdivia, we meet with the same mica-schist, exhibiting nearly the same mineralogical passages as in the Chonos Archipelago, often, however, becoming more ferruginous, and containing so much feldspar as to pass into gneiss.The folia were generally well defined; but nowhere else in South America did I see them varying so much in direction: this seemed chiefly caused by their forming parts, as I could sometimes distinctly trace, of large flat curves:
nevertheless, both near the settlement and towards the interior, a N.W.and S.E.strike seemed more frequent than any other direction; the angle of the dip was generally small.At Concepcion, a highly glossy clay-slate had its cleavage often slightly curvilinear, and inclined, seldom at a high angle, towards various points of the compass: but here, as at Valdivia, a N.W.and S.E.strike seemed to be the most frequent one.