line.For a space of 300 miles on the shores of the Chonos and Chiloe Islands, we have seen that the foliation seldom deviates more than a point of the compass from a N.19 degrees W.and S.19 degrees E.strike.As in the case of cleavage, the angle of the dip in foliated rocks is generally high but variable, and alternates from one side of the line of strike to the other side, sometimes being vertical: in the Northern Chonos Islands, however, the folia are inclined almost always to the west; in nearly the same manner, the cleavage-laminae in Southern Tierra del Fuego certainly dip much more frequently to S.S.W.than to the opposite point.In Eastern Banda Oriental, in parts of Brazil, and in some other districts, the foliation runs in the same direction with the mountain-ranges and adjoining coast-lines: amongst the Chonos Islands, however, this coincidence fails, and I have given my reasons for suspecting that one granitic axis has burst through and tilted the already inclined folia of mica-schist: in the case of cleavage, the coincidence between its strike and that of the main stratification seems sometimes to fail.(Cases are given by Mr.Jukes in his "Geology of Newfoundland" page 130.) Foliation and cleavage resemble each other in the planes winding round concretions, and in becoming tortuous where veins of quartz abound.(I have seen in Brazil and Chile concretions thus enfolded by foliated gneiss; and Macculloch "Highlands"volume 1 page 64, has described a similar case.For analogous cases in clay-slate, see Professor Henslow's Memoir in "Cambridge Philosophical Transactions" volume 1 page 379, and Macculloch's "Classification of Rocks"page 351.With respect to both foliation and cleavage becoming tortuous where quartz-veins abound, I have seen instances near Monte Video, at Concepcion, and in the Chonos Islands.See also Mr.Greenough's "Critical Examination" page 78.) On the flanks of the mountains both in Tierra del Fuego and in other countries, I have observed that the cleavage-planes frequently dip at a high angle inwards; and this was long ago observed by Von Buch to be the case in Norway: this fact is perhaps analogous to the folded, fan-like or radiating structure in the metamorphic schists of the Alps, in which the folia in the central crests are vertical and on the two flanks inclined inwards.(Studer in "Edinburgh New Philosophical Journal"volume 23 page 144.) Where masses of fissile and foliated rocks alternate together, the cleavage and foliation, in all cases which I have seen, are parallel.Where in one district the rocks are fissile, and in another adjoining district they are foliated, the planes of cleavage and foliation are likewise generally parallel: this is the case with the feldspathic homogeneous slates in the southern part of the Chonos group, compared with the fine foliated mica-schists of the northern part; so again the clay-slate of the whole eastern side of Tierra del Fuego cleaves in exactly the same line with the foliated gneiss and mica-slate of the western coast;other analogous instances might have been adduced.(I have given a case in Australia.See my "Volcanic Islands.")With respect to the origin of the folia of quartz, mica, feldspar, and other minerals composing the metamorphic schists, Professor Sedgwick, Mr.
Lyell, and most authors believe, that the constituent parts of each layer were separately deposited as sediment, and then metamorphosed.This view, in the majority of cases, I believe to be quite untenable.In those not uncommon instances, where a mass of clay-slate, in approaching granite, gradually passes into gneiss, we clearly see that folia of distinct minerals can originate through the metamorphosis of a homogeneous fissile rock.(I have described in "Volcanic Islands" a good instance of such a passage at the Cape of Good Hope.) The deposition, it may be remarked, of numberless alternations of pure quartz, and of the elements of mica or feldspar does not appear a probable event.(See some excellent remarks on this subject, in D'Aubuisson's "Traite de Geog." tome 1 page 297.Also some remarks by Mr.Dana in "Silliman's American Journal" volume 45 page 108.)In those districts in which the metamorphic schists are foliated in planes parallel to the cleavage of the rocks in an adjoining district, are we to believe that the folia are due to sedimentary layers, whilst the cleavage-laminae, though parallel, have no relation whatever to such planes of deposition? 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? Are we to admit that in the northern part of the Chonos Archipelago, mica-slate was first accumulated in parallel horizontal folia to a thickness of about four geographical miles, and then upturned at an angle of forty degrees; whilst, in the southern part of this same Archipelago, the cleavage-laminae of closely allied rocks, which none would imagine had ever been horizontal, dip at nearly the same angle, to nearly the same point?
