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I.—The Metamorphosis of the Lizard Gabbros1

Published online by Cambridge University Press:  01 May 2009

J. J. H. Teall
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If we take a general view of the present position of geological science, we are struck by the fact that, although there is substantial agreement amongst geologists on matters relating to the origin of the rocks usually designated as aqueous and igneous, the greatest diversity of opinion prevails with regard to the circumstances under which the so-called metamorphic rocks have been produced. Every fragment of evidence calculated to throw light on the origin of these rocks, therefore, deserves the most careful consideration. Of recent years special attention has been directed to the effects of mechanical energy in modifying the mineralogical and structural characters of rocks originally formed by aqueous and igneous agencies; and a suspicion has been aroused that it is in this direction that we must look for a solution of many of the problems connected with the origin of the crystalline schists. A visit to the Lizard Peninsula of Cornwall during the present summer has convinced me of the immense importance of this view so far as that district is concerned. That portion of the peninsula which lies south of a line drawn from Porthalla on the east to Polurrian Cove on the west is formed.partly of igneous rocks—such as gabbro, greenstone, serpentine, and granite—and partly of crystalline schists. The igneous rocks, in certain places, become foliated and sohistose and sometimes show a definite banding due to a variation in the relative proportions of the different constituents. In other words they present characters which are usually regarded as distinctive of the crystalline schists. There is, moreover, evidence to show that these characters are mainly the result of a yielding to earth-pressure subsequent to the consolidation of the original rock. At the present moment, having just returned from the district, I am unable to treat the subject from a general point of view with any prospect of success; but it has occurred to me that some details with regard to one of the rocks may not be without interest to members of the Association.

Type
Original Articles
Information
Geological Magazine , Volume 3 , Issue 11 , November 1886 , pp. 481 - 489
Copyright
Copyright © Cambridge University Press 1886

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References

page 481 note 2 Trans. Geol. Society of Cornwall, vol. iv. p. 330Google Scholar.

page 481 note 3 Ibid. vol. i. p. 36.

page 481 note 4 Report on the Geology of Devon and Cornwall, 1839.

page 482 note 1 Quart. Journ. Geol. Soc., vol. xxxiii. 1877, p. 884Google Scholar.

page 483 note 1 There is some confusion about the naming of this locality. The small bay to the north of which the gabbro is exposed is named the Balk on the one-inch map and Parn Voose. on the twenty-five inch map. The inhabitants of this district say that this bay is called Pen Voose and that the place where the life-boat is kept, otherwise known as Church Cove, is Parn Voose. As the maps do not agree, I have followed the inhabitants.

page 483 note 2 Saussurit, Ueber, Zeit. f. Krystallographie, vol. vii. p. 234Google Scholar.

page 484 note 1 A saussurite composed of zoisite and albite occurs in Sangomore Bay near Durness, Sutherlandshire. Under the microscope this is seen to consist of minute colourless prisms embedded in a colourless felspathic matrix. The prisms possess a high refractive index, weak double refraction, cross-jointing at intervals, and give straight extinction. Small fragments of the felspar with which they are associated may occasionally be obtained and when tested in the Bunsen's burner give the flame colouration and fusibility of albite. In the mass this saussurite is white or a very pale pink. It contains hornblende and is associated with a felspathic gabbro in which the diallage has been converted into uralite and actinolite.

page 484 note 2 Studien an metamorphischen Eruptiv- und Sedimentgesteinen, Jahrbuch d. k. preuss. geol. Landesanstalt für 1883 und 1884.

page 488 note 1 Die Fossilien führenden krystallinischen Schiefer von Bergen in Norwegen, Leipzig, 1883Google Scholar.

page 488 note 2 Die Entstehung der altkrystallinischen Schiefergesteinen, Bonn, 1884, p. 190Google Scholar.

page 488 note 3 Tsch. Min. Mitth, Band VII. (1885), p. 75Google Scholar.