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I.—Foliation and Metamorphism in Rocks
Published online by Cambridge University Press: 01 May 2009
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The application of the microscope to petrology made it possible to I investigate effectively the history of the foliated rocks and of metamorphism. This was done, as it happens, only a very few years before I began any special study of such rocks, so that the account, of my own work almost corresponds with that of the general progress in a knowledge of them. I was led, after some preliminary efforts, into investigating two distinct problems, each of which, as I soon discovered, presented rather exceptional difficulties. These were the pre-Carboniferous rocks of Charnwood Forest and the serpentines of the Lizard. The problem involved in the one was how far some of them were or had been igneous in origin, lavas, tuffs, and agglomerates, or were stratified rocks, which by pressure and mineral changes had been so altered as to be indistinguishable from some of the former. The other introduced the question of the origin of serpentine, about which in 1873 the utmost uncertainty existed. This, from its associations, led on to investigating the nature and origin of gneisses and schists, so that the history of my own studies during the last forty-five years happens to illustrate. some important aspects of the progress made during that period.
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References
page 196 note 1 The following remarks appeared in Rocks, Classified and Arranged (B. von Cotta; translated by P. H. Lawrence and published in 1866 with author's preface of same date). After stating that serpentine is “probably the product of the metamorphosis of some other rock”, he continues (p. 317): “In some places … its transmutation from other rocks is very evident, as, for instance, from gabbro at Siebenlehn, near Freiberg; from dykes of granite traversing serpentine rocks near Böhrigen and Waldheim in Saxony, where the main serpentine itself is not improbably a transmuted granulite; from chloriteschist at Zell in the Fichtelgebirge, where the change does not appear to be yet complete; and from gneiss (probably) or an eclogite rock in the gneiss at Zōblitz in the Erzgebirge.”
page 197 note 1 Q.J.G.S. xlviii, p. 390.
page 197 note 2 The story of this “Comedy of Errors” is told in “Plant Stems in the Guttannen Gneiss”, Geol. Mag. 1900, p. 215.
page 197 note 3 J. Macculloch died in 1835, H. T. De la Beche in 1855, but the clouds were gathering again before the latter date. An excellent account of the difficulties which had to be encountered by those who were living and working in the earlier half of the nineteenth century is given by Sir A. Geikie in The Founders of Geology, ch. viii.
page 197 note 4 He began this work, for some years little appreciated, about 1850.
page 198 note 1 See Manual of Geology, Jukes, J. B., 1872 (ed. Geikie, A.), p. 142Google Scholar.
page 199 note 1 Figures in heavy type refer to the bibliography at the end of the paper.
page 200 note 1 Similar gneissoid rocks occur on the southern border of the Highland complex from near Stonehaven northward for some miles.
page 200 note 2 Certain minerals, such as hornblende, biotite, chloritoid, ottrelite, dipyr, couseranite, and a plagioclase felspar, seem to form with comparative ease, as will presently be described, in some crushed gneisses and hornblendic rocks.
page 200 note 3 The advocates of this notion appear to have forgotten that normal crystalline quartz is a frequent constituent of felsites, rhyolites, etc.
page 202 note 1 The pressure-modified quartzites of the Scotch Highlands, such as those of Glendhu in Sutherland, present the nearest resemblance to a true quartzschist.
page 202 note 2 Descriptions of these schists, together with analyses quoted from an article by Dr. Grubenmann (Mitth. Thurg. naturf. Ges., Heft viii, 1888), are given in a paper on “Crystalline Schists and their Relation to Mesozoic Rocks in the Lepontine Alps” (Q.J. 1890, pp. 187–236). The garnets appear to be an impure representative of the alumina-lime variety. They are sometimes about one-third of an inch in diameter.
page 202 note 3 They had already been analysed (see Q.J.G.S. 1890, p. 233) by Fritsch (Beiträge zur Geol. Karte der Schweiz, Lief. xv, p. 127) and the impossibility of identifying his knoten and prismen with garnet and staurolite implicitly demonstrated.