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| Keywords: The Quarterly journal of the Geological Society of London (12733807025).jpg J W JUDD ON THE SECOXDA EY EOCKS OP SCOTLAND 293 <br> composed of hornblendic granite passing by insensible gradations <br> into ordinary granite on the one band and into syenite granite on <br> tbe other The normal character of this rock is that of an aggregate <br> of white orthoclase and oligoclase felspar with colourless quartz and <br> hornblende the latter being usually in part replaced by varying <br> proportions of lepidomelan ; and the whole mass is rendered beauti- <br> fully porphyritic by the dissemination through it of fine crystals of <br> orthoclase felspar of a pink colour Locally however it exhibits <br> many variations from the typical character It is traversed too in <br> every direction by veins of various size composed usually of <br> granite of finer grain of euritic felsite or of crystallized quartz and <br> felspar The granitic rocks constitute those great spurs with sharp <br> summit-ridges and steeply sloping sides which divide the deep corries <br> that form so striking a feature in the Beinn Nevis group <br> Let us now ascend the central peak and examine its structure <br> Some distance above the well-known lake which lies in a hollow <br> upon the shoulders of the mountain a remarkable change is found <br> to take place in the character of the granitic mass ; it becomes much <br> finer-grained and as we still ascend we find the hornblende and <br> mica gradually disappearing till in the end the rock becomes a finely <br> granular felsite of a pale red colour and often more or less porphy- <br> ritic in structure <br> The highest portion of the mountain however is composed of a <br> mass of rocks of totally different character Instead of the pale red <br> granites eurites and felsites we find dark-blue grey greeenish <br> and purplish felstones ; and associated with these are enormous <br> masses of volcanic agglomerate composed for the most part of <br> angular fragments of all sizes of felspathic materials heaped <br> together in the wildest confusion and compacted into masses of <br> great solidity and hardness <br> The confusion produced by the ever descending fragments which <br> cover the upper slopes of the mountain and constitute such im- <br> pressive evidence of the potency of atmospheric agencies in the <br> work of denudation is inimical to a complete study of the relations <br> of the rocks forming the summit of the mountain ; but an atten- <br> tive examination both of these slopes and of the more accessible of <br> the precipices surrounding the corries shows that the felstones form <br> great sheets sometimes exhibiting a rudely columnar structure <br> and that between them lie the enormous masses of volcanic agglo- <br> merate the whole being traversed by innumerable felstone dykes <br> From the same cause namely the abundance of debris on the <br> higher slopes of the mountain the relations of the underlying <br> felsites and granite to the great cap composed of felstones and <br> agglomerates is not at first sight very apparent Careful study <br> however shows that the former rocks send off veins into the latter ; <br> and the abundance of the fragments of the granitic rocks above the <br> level at which the lavas and agglomerates commence shows that <br> these veins are by no means few in number <br> With regard both to the felstone lavas and the associated agglo- <br> merates the changes produced by the chemical action that has taken <br> Q J G S No 119 t 35766188 110599 51125 Page 292 Text v 30 http //www biodiversitylibrary org/page/35766188 1874 Geological Society of London Biodiversity Heritage Library The Quarterly journal of the Geological Society of London v 30 1874 Geology Periodicals Smithsonian Libraries bhl page 35766188 dc identifier http //biodiversitylibrary org/page/35766188 smithsonian libraries Information field Flickr posted date ISOdate 2014-02-24 Check categories 2015 August 26 CC-BY-2 0 BioDivLibrary https //flickr com/photos/61021753 N02/12733807025 2015-08-26 19 57 43 cc-by-2 0 PD-old-70-1923 The Quarterly journal of the Geological Society of London 1874 Photos uploaded from Flickr by Fæ using a script | ||||
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