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Many attempts have been made to trace a general order of succession or superposition in the members of this family; clay-slate, for example, having been often supposed to hold invariably a higher geological position than mica- schist, and mica-schist to overlie gneiss. But although such an order may prevail throughout limited districts, it is by no means universal.
Frequently, between the granite and the hornblende-slate above- mentioned, grains of mica and crystalline feldspar appear in the schist, so that rocks resembling gneiss and mica-schist are produced. Fossils can rarely be detected in these schists, and they are more completely effaced in proportion to the more crystalline texture of the beds, and their vicinity to the granite.
In addition to partial chambers made with the pick-axe, one sees here and there a series of stairs cut out of the mica-schist. The strength of the burg made it a place of refuge for numerous families in the Albigeois, who had retreats upon these rocks to which they repaired in time of danger. All that made up the grandeur and importance of the place has passed away.
There can be no doubt in these cases that strata once composed of mud, sand, and gravel, or of clay, marl, and shelly limestone, have for the distance of several yards, and in some instances several hundred feet, been turned into gneiss, mica-schist, hornblende-schist, chlorite- schist, quartz rock, statuary marble, and the rest.
In Chiloe, a great formation of mica-schist strikingly resembles that of 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.
Continuing on the same line I crossed another small watercourse falling north-east; and beyond it were hills of mica-schist and quartz, which sloped rather boldly to the southward. We then entered one of the finest tracts of forest land I ever saw. It was there three miles in width, and bounded on the south by another low hill of quartzose gravel, the soil of which was indifferent.
This hypogene rock, called by the earlier geologists PRIMARY LIMESTONE, is sometimes a white crystalline granular marble, which when in thick beds can be used in sculpture; but more frequently it occurs in thin beds, forming a foliated schist much resembling in colour and arrangement certain varieties of gneiss and mica-schist.
There is in that locality a blue limestone foliated by the intercalation of small plates of white mica, so that the rock is often scarcely distinguishable in aspect from gneiss or mica-schist.
On re-ascending from Punkabaree, the rocks gradually appear more and more dislocated, the clay-slate less so than the quartz and mica-schist, and that again far less than the gneiss, which is so shattered and bent, that it is impossible to say what is in situ, and what not.
Beds of pure quartz also occur in this formation. In some districts, garnets in regular twelve-sided crystals form an integrant part of mica-schist. This rock passes by insensible gradations into clay-slate. This rock sometimes resembles an indurated clay or shale. It is for the most part extremely fissile, often affording good roofing-slate.
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