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Updated: May 1, 2025
Agnes Beacon the coast is largely composed of clay-slates, or killas, presenting much desolate grandeur; the slate showing the jagged scars of its unending resistance to oceanic forces. At Cligga Head this slate is blended with decomposed hard granite. Off the shore, about two miles out, rise the two isolated rocks known as the Man and his Men sometimes also called the Cow and her Calf.
In the gneiss round Valparaiso the strike of the foliation is very variable, but I think about N. by W. and S. by E. is the commonest direction; this likewise holds good with the cleavage of the altered feldspathic clay-slates, occasionally met with on the coast for ninety miles north of Valparaiso.
These 'hard-heads, which try the hammer, show a revetment of cellular iron upon a solid core of greenstone and bluish trap. Some fragments not a little resembled the clay-slates of the Brazilian gold-mines. Such was the concession which we named São João do Principe. Presently the chief, Mrá Kwámi, announced to us that we had reached the northern boundary-line of the estate.
Such clay-slates, in fact, are variable in composition, and sometimes alternate with fossiliferous strata, so that they may be said to belong almost equally to the sedimentary and metamorphic order of rocks.
Occasionally it derives a shining and silky lustre from the minute particles of mica or talc which it contains. It varies from greenish or bluish-grey to a lead colour; and it may be said of this, more than of any other schist, that it is common to the metamorphic and fossiliferous series, for some clay-slates taken from each division would not be distinguishable by mineral characters alone.
Marble unlike sandstones, clay-slates, and basalts, which are kindred to the earth and the elements, and find themselves at home in any situation, all things making friends with them, mosses, lichens, ivies is a dead, cold material, and does not harmonise with surrounding circumstances.
In the common salt by which strata are often largely impregnated, as in Patagonia, much soda is present, and potash enters largely into the composition of fossil sea-weeds, and recent analysis has also shown that the carboniferous strata in England, the Upper and Lower Silurian in East Canada, and the oldest clay-slates in Norway, all contain as much alkali as is generally present in metamorphic rocks.
These spheres are disposed in layers parallel to the foliation of the gneiss: and are the result of a metamorphic action of great intensity, effecting a complete rearrangement and crystallization of the quartz and mica in parallel planes, whilst the felspar is aggregated in spheres; just as in the rearrangement of the mineral constituents of mica-schists, the alumina is crystallized in the garnets, and in the clay-slates the iron into pyrites.
It is, indeed, more common to meet with some variety of clay-slate forming the uppermost member of a metamorphic series than any other rock; but this fact by no means implies, as some have imagined, that all clay-slates were formed at the close of an imaginary period when the deposition of the crystalline strata gave way to that of ordinary sedimentary deposits.
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