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III.—On Certain Rock-Structures, as Illustrated by Pitchstones and Felsites in Arran
Published online by Cambridge University Press: 01 May 2009
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In a paper read on February 23rd, 1876, before the Geological Society of London, I endeavoured to prove that the peculiar laminated or fissile structure, common in certain igneous rocks, was due to contraction, like the associated ordinary joint-structures. In the discussion which followed, an objection was made to my having quoted certain phonolites from Auvergne, in support of my theory, because it was well known that the fissile structure in the typical phonolites of Hungary was due to change in mineral composition, and so the result of a banded structure in the rock. That this was not the case with those Auvergne phonolites, I had already convinced myself by examination, both in the field and with the microscope, and I had nowhere asserted that the only way in which rocks could become fissile was by the particular cause which I had advanced; for, although I only knew the Hungary rocks from hand-specimens, I was aware of a somewhat similar structure in certain British rocks. I thought I remembered also one or two cases in these where the fissile and the banded structure were co-existent and independent, but as the criticism came from a quarter not to be neglected, I determined to re-examine these cases. This I have done on occasion of short visits to Arran in the past and present year, during which I have obtained materials which will, I think, further illustrate this, and throw some light on other rock structures. The points, then, upon which I shall touch in the present paper are the relation (I.) of fissile and banded structures; (II.) of banded, spherulitic, and perlitic structures.
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References
Page 499 note 1 Q.J.G.S. vol. xxxii. p. 140.Google Scholar
Page 500 note 1 Mentioned by Bryce, Geology of Arran, p. 72.
Page 500 note 2 The base of the felsite near the basalt has a vitrified appearance and has a faint indication of banding.
Page 502 note 1 See MrAllport's, S. description and plates, Geol. Mag. 12. 1. Vol. IX. p. 1. See also p. 537.Google Scholar
Page 502 note 2 Monthly Microscopic Journal, 1876, p. 176. The author kindly refers to my work so far as regards basalt only; the reason being that I was obliged, in reading, greatly to condense the paper, and dismissed perlite in one or two words, which escaped his notice.
Page 502 note 3 Quart. Journ. Geol. Soc. vol. xxxiii. p. 440.Google Scholar
Page 502 note 4 Thus: the strain elongates oval masses and tends to draw them out into ropy filaments. Further, if a mass consisted of particles of various sizes, motion under strain would have a tendency to pack like with like.
Page 503 note 1 See MrAllport, S., Geol. Mag. 12. 1. Vol. IX. p. 541.Google Scholar
Page 503 note 2 Geol. Mag. 12. 1. Vol. IX. Pl. I. Fig. 2.Google Scholar
Page 504 note 1 I am not aware that the locality where this occurs in situ is known. It is the same rock as is described by Mr. , Allport from a boulder occurring near Benan, W. (Geol,. Mag. 12. 1. Vol. IX. p. 537).Google Scholar
Page 504 note 2 I have observed in numerous instances in trachytes, felsites, etc., that the formation of the larger crystals of felspar and some other minerals seems anterior to the setting up of microlithic structure. Can it be that in many cases these have been formed in some prior cooling of the rock, and that the last elevation of temperature previous to the ejection of the rock only fused the ground-mass? Except on some such theory it is very hard to explain such cases as are found in some of the coarser leucite lavas of Vesuvius.
Page 506 note 1 Geol. Mag. 12. 1. Vol. IX. p. 541.Google Scholar
Page 508 note 1 Geol. Mag. 12. I. Vol. IX. p. 541.Google Scholar
Page 508 note 2 Exactly the same structure is seen in the felsite from near Drumadoon, mentioned above.
Page 509 note 1 I am indebted to a college friend for the following additional evidence. The spherulitic felsite agrees in composition more closely with the other felsite (allowing for its state) than with the pitchstone; the last having at least 2 per cent, less SiO2. Again, when the powder of all three is heated to redness for about 10 m., that of the two felsites does not cohere, and afterwards cannot be distinguished in colour—being an ochreous grey; while that of the pitchstone coheres and is brownish stone-grey. Again, on fusing both the spherulitic felsite and the pitchstone with (K. Na.) CO3, the former assumes a yellowish-green tinge, and the latter a rather clear bright green, showing there is more Mn or less Fe in it. The pitchstone also (though the coarser powder) decomposed readily with the fusing mixture, while both the felsites left a residue, which required prolonged fusion.
Page 509 note 2 As when we find them associated with the brown glass of pitchstones.
Page 510 note 1 See Quart Journ. Geol. Soc. vol. xxxi. p. 152.Google Scholar
Page 510 note 2 loc. cit. p. 153.
Page 511 note 1 With this compare the “veined structure” in glacier ice, as explained by Prof. Tyndall, Glaciers of the Alps, p. 376.