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On a peculiar variety of Hornblende from Mynydd Mawr, Carnarvonshire

Published online by Cambridge University Press:  14 March 2018

T. G. Bonney
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Extract

Mynydd Mawr is a conspicuous hill with a craggy face towards the north, and a well-marked corrie (Cwm Du) on that side. It lies to the south-east of Moel Tryfan and the west of Llyn Cwellyn (Carnarvonshire). On the Geological Survey map it is represented as a somewhat rounded oblong patch of felstone, about 1⅜ mile from east to west, and a mile, or a little more, from north to south, intrusive in Lower Silurian (Ordovieian) rocks, which are probably Arenig. I passed some years since to the north of the above-named face, and collected specimens from erratics which were plentifully strewn over the lower land. On microscopic examination I found them to contain a very puzzling and remarkable mineral which I have long intended to describe, but kept deferring this in hope of being able to revisit the place to obtain more specimens for examination, and by taking these from rock in situ, to prevent any question arising as to the true source of the rock.

Type
Research Article
Information
Mineralogical magazine and journal of the Mineralogical Society , Volume 8 , Issue 37 , October 1888 , pp. 103 - 107
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1888

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References

page 108 note 2 The unusual quantity of snow impeded him in his work, and, unfortunately, a letter which I had written to him failed to reach him till too late.

page 104 note 1 Mr. Harker speaks of larger specimens, about a quarter of an inch long.

page 104 note 2 I do not think it can be tourmaline, as suggested by Mr. Harker.

page 104 note 3 The acicular mineral described above is also included, so it must belong to a, very early stage of consolidation.

page 105 note 1 The two, the green and the blue-black (commoner), are so closely associated and similar in all but colour that I feel sure they must be nearly related. Mr. Harker describes the latter as tourmaline. Formerly I held the same opinion, but am now convinced that the rock does not contain this mineral, but two varieties of hornblende.

page 106 note 1 See also Roaenbusch, Mikros. Physiog. Massig. Gestein. p. 312. A slide, however cut from a crystal of Norwegian arfvedsonite, shown me by Prof Judd, does not show this intense blue colour.