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Nature volume 13, pages 379380 | Download Citation




Geological Society, Feb. 23.-Prof. P. Martin Duncan, F.R.S., president, in the chair.-The Rev. David Charles, D.D., Thomas Musgrave Heaphy, C.E., William Smethurst, Edward Horatio W. Swete, M. D., and John Thomas Young were elected Fellows, and Prof. Joseph Gosselet, of Lille, a Foreign Correspondent of the Society.-The following communications were read:-On the greenstones of Western Cornwall, by Mr. John Arthur Phillips, F.C. S. In this paper the author brought forward evidence to show that the so-called “greenstones “of Penzance really belong chiefly to the following three classes:-a, Gabbros or Dolerites, in which the originally constituent minerals are either to a great extent unchanged, or, sometimes, almost entirely represented by pseudo-morphic forms, b. Killas, or ordinary clay-slates, c. Highly basic hornblendic rocks, exhibiting a tendency to break into thin plates; these under the microscope present the appearance of metamorphosed slates. Slaty rocks of a character intermediate between b and c also occur. In the Cape Cornwall district the “greenstones“are chiefly hornblendic slates, sometimes with veins or bands of garnet, magnetite, or axiniie. The rocks near the Gurnard's ?-ad are almost identical with those of Mount's Bay. The crystalline pyroxenic rocks and metamorphic slates of the St. Ives district exactly resemble those of Penzance. The greenstones between St. Erth and St. Stephen's are probably altered ash-beds or hardened hornblendic slates; unlike the hornblendic and augitic rocks of the other districts, they do not occur in the immediate vicinity of granite, but elvan courses are always found near them. The percentage of silica in the two series of rocks is nearly constant; the hornblende slates contain about 10 per cent less silica than the crystalline pyroxenic rocks, and there is an excess of iron oxides to nearly the same extent, their composition in other respects being very similar. The Killas is an acidic rock of essentially different chemical composition.-On columnar, fissile, and spheroidal structure, by the Rev. T. G. Bonney. Some of the above structures have comparatively recently been discussed by Mr. Mallet and Prof, J. Thomson. Both these authors agree in attributing columnar 1 structure to contracuon due to loss of heat while cooling, but differ in their explanation of cross jointing and spheroidal structure. In this paper it is sought to show that the principle proved by Mr. Mallet to be the explanation of the columnar structure is capable of a wider application. After a brief notice of some instances of columnar structure, the author described cases of a fissile structure seen in certain igneous rocks (especially in the Auvergne phonolites), closely resembling true cleavage, and often mistaken for it; also the tabular jointing of rocks; a peculiar form of this, where most of the segments are of a flattened convexo-concave form; spheroidal structure and cup-and-ball structure. He showed by examples that Prof. Thomson's explanation of spheroidal structure was inadequate, and gave reasons for considering all these structures to be due to contraction. He also discussed more particularly the cup-and-ball structure, giving reasons for thinking that the spheroidal and the horizontal fissures were often to some extent independent of each other.

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