File:The Quarterly journal of the Geological Society of London (13937107491).jpg
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[edit]DescriptionThe Quarterly journal of the Geological Society of London (13937107491).jpg |
250 MISS C. A. RAISIN OX SOME belonging to that formation; but the nearest fossiliferous outcrop recorded- is distant two or three miles. The country around is drift-covered, but I found some indications of stratigraphical position near the streamlet which flows to Llym-gwyn. Felsite of the usual type occurs close by the cottages, although further south, just beyond the railway, is one outcrop, which seems probably an ash, consisting of broken felstone and some slate. Walking northward for about thirty yards, along the path east of the stream, we pass over a small exposure of a peculiar black and white rock, which has a squeezed look. The milk-white fragments of felstone exhibit, under the microscope, traces of banding, possibly fluidal ; they are, at places, spherulitized, and contain abundant microliths or globulites. Slat. T fragments are entangled, which are often rounded, and some are beginning to crack along curving boundaries. This rock seems to be a volcanic agglomerate, which has become schistose from pressure. Along the next 200 yards, well-banded ashy and slaty beds occur here and there, in which I found a few small fossils. These were several squeezed examples of Trinudeus concentricus, small specimens of Leptcena sericea, a distorted Orthis elega?itula, and other young forms of Ortliis (sp. ?). This assemblage has a Bala facies, and Mr. Etheridge, who very kindly identified some of the specimens for me, gave his opinion, from the fossil and lithological characters, that the strata may be taken as belonging, in all probability, to that formation. The microscope shows quartz and plagioclase felspar in small angular fragments and a minute secondary mineral ; the meshwork of dark cleavage-planes crosses the beds almost at right angles. Some 30 yards succeed, occupied by a felspathic rock, part of which forms two small craglets overlooking the stream. The beds, although uniform, are well marked, dipping about ET.N'.E. at an angle of 45° or 50°. They are crossed by a cleavage similar to that of the slaty and ashy strata, and the subsequent pressure, which thus modified the rock, doubtless caused the fracturing of the included felspar. The appearance of the slide is suggestive, as Professor Bonney pointed out to me, of a very porphyritic rhyolite, now devitrified and crushed. The next outcrop is of a compact pale grey rock, which proves to be a felstone, enclosing small fragments of slate, quartz, and felspar. The fluidal layers are much contorted, and exhibit a deposit of a minute secondary mineral. Thus the Pen-y-chain felsites seem to pass upward into a series of agglomerates and lavas, with interbedded slates and grits, including Bala fossils. If the igneous strata of the Snowdon district once extended over the Lleyn, as suggested in the Survey Memoir *, the Pen-y-chain mass may be a remnant of such volcanic accumu- lations. The small area of Careg-y-defaid does not need separate detailed description. • A fine fluidal structure in a porphyritic rock is well The Geology of tfortk Wales, pp. 218,221. |
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Source | https://www.flickr.com/photos/biodivlibrary/13937107491 | ||
Author | Geological Society of London | ||
Full title InfoField | The Quarterly journal of the Geological Society of London. | ||
Page ID InfoField | 36940043 | ||
Item ID InfoField | 113696 (Find related Wikimedia Commons images) | ||
Title ID InfoField | 51125 | ||
Page numbers InfoField | Page 248 | ||
Names InfoField | NameFound:Orthis NameConfirmed:Orthis EOLID:4333134 NameBankID:4270859 | ||
BHL Page URL InfoField | https://www.biodiversitylibrary.org/page/36940043 | ||
Page type InfoField | Text | ||
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Flickr tags InfoField |
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Flickr posted date InfoField | 21 April 2014 | ||
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26 August 2015
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current | 05:49, 26 August 2015 | 1,154 × 2,105 (466 KB) | Fæ (talk | contribs) | == {{int:filedesc}} == {{BHL | title = The Quarterly journal of the Geological Society of London. | source = http://www.flickr.com/photos/biodivlibrary/13937107491 | description = 250 MISS C. A. RAISIN OX SOME <br> belonging to... |
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