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Updated: June 1, 2025
There are, however, some that are of undoubted sedimentary origin, but highly metamorphosed. The general direction of foliation is a few degrees south of east, subject, of course, to many local changes.
Fox, showing the influence of electrical currents in producing a structure like that of cleavage, and notwithstanding the apparently inexplicable variation, both in the inclination of the cleavage-laminae and in their dipping first to one side and then to the other side of the line of strike, lead me to suspect that the planes of cleavage and foliation are intimately connected with the planes of different tension, to which the area was long subjected, AFTER the main fissures or axes of upheavement had been formed, but BEFORE the final consolidation of the mass and the total cessation of all molecular movement.
Throughout this district, the cleavage of the clay-slate and marble the foliation of the gneiss and the quartz the stratification or alternating masses of these several rocks and the range of the hills, all coincide in direction; and although the country is only hilly, the planes of division are almost everywhere very highly inclined or vertical.
On this view, how can we reconcile the vastness of the areas over which the strike of the foliation is uniform, with what we see in disturbed districts composed of true strata: and especially, how can we understand the high and even vertical dip throughout many wide districts, which are not mountainous, and throughout some, as in Western Banda Oriental, which are not even hilly?
Owing to the strong weathering action this thickness of quartzite is doubtless much less than it was originally. Forty-six miles above Seal Lake an exposure of phyllite was seen, the same in every respect as the one east of Seal Lake, just mentioned. The general direction of foliation is S. 70 degrees E. and the dip 70 degrees.
At Los Arenales, low down on the eastern flank, the mica-slate is traversed by several closely adjoining, broad dikes, parallel to each other and to the foliation of the schist. The dikes are formed of three different varieties of rock, of which a pale brown feldspathic porphyry with grains of quartz was much the most abundant.
The first palace beyond the bridge, now a decaying congeries of offices, has very rich decorative stone work, foliation and festoons. It was once the head-quarters of the Camerlenghi, the procurators-fiscal of Venice.
At Bahia, the gneiss rocks are similarly decomposed, with the upper parts insensibly losing their foliation, and passing, without any distinct line of separation, into a bright red argillaceous earth, including partially rounded fragments of quartz and granite.
The original foliation seems to have been cut away, and the intermediate mullions extended to the points of the two lights. This may have been done with a view to economy in reglazing the window. The modern window is shown on page 37. A small chamber to contain the hydraulic apparatus for the organ has recently been added to the east side of the south transept.
I shall in the few following remarks on the rocks of Chile allude exclusively to their foliation and cleavage.
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