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| Keywords: The Quarterly journal of the Geological Society of London (12734285014).jpg 282 J W JTJDD ON THE SECONDARY ROCKS OF SCOTLAND <br> will serve to illustrate the peculiar relations of the several rocks in <br> these cases <br> The rocks on which the volcanic series of Lorn usually rests are <br> black clay-slates abounding in fine cubical crystals of pyrites and <br> often traversed by numerous veins of quartz This formation is <br> perhaps tbe highest member preserved to us of that great series of <br> Lower Silurian rocks which bent into endless folds aud greatly me- <br> tamorphosed occupies so large a portion of the Scottish Highlands <br> At Seil and Easdale it is extensively worked for roofing-slates ; as <br> yet unfortunately it has not yielded any fossils ; but it exhibits in <br> the island of Seil interbedded igneous rocks apparently of contem- <br> poraneous character <br> The only point at which the series of rocks so well displayed in <br> Lorn is seen in juxtaposition with the Mesozoic sedimentary rocks <br> and the Tertiary lavas is in the south-eastern part of Mull Here <br> unfortunately among the wonderfully disturbed and greatly meta- <br> morphosed rocks which are exposed only in precipitous and altogether <br> inaccessible cliffs I have sought in vain for any simple section illus- <br> trating the relations of these three series of deposits Nevertheless <br> after carefully tracing the positions of such masses as can be reached <br> and studied there appears to be no room for doubt that the various <br> members of the Mesozoic series rest indifferently upon the denuded <br> lavas of Lorn and the older rocks and that they are themselves <br> covered unconformably by the Tertiary volcanic rocks <br> 4 Succession of Rocks in Lorn � The series of rocks which we have <br> been describing has usually been represented as consisting of beds of <br> conglomerate and sandstone at the base overlain by a great mass of <br> trap rocks A careful examination of the district however <br> proves that its structure is by no means so simple as this statement <br> would imply The outpouring of the great lava streams was in part <br> at least contemporaneous with as well as subsequent to the depo- <br> sition of the conglomerates and sandstones This is proved by the <br> alternation of the trap rocks with the conglomerates and sand- <br> stones and by the fact that the materials of the latter are to a <br> great extent derived from the former Good examples of the alter- <br> nations of the traps with the sandstones and conglomerates are to be <br> seen near Dunolly <br> At the base of the whole series of the Lorn rocks and in imme- <br> diate contact with the subjacent slates is often found a breccia of <br> very interesting character It is wholly composed of perfectly an- <br> gular fragments sometimes of considerable size of the slate and <br> quartz rocks on which it rests ; this breccia exhibits no trace of <br> stratification or of its materials having been sorted or acted upon <br> by water It sometimes forms masses of considerable thickness <br> which appear to have been accumulated upon old terrestrial surfaces <br> of the slate rocks by purely subaerial agencies <br> The conglomerates of the Lorn series present very remarkable <br> characters which are familiar to all who have examined the pictur- <br> esque cliffs on either side of the beautiful Bay of Oban They are <br> made up of blocks of very various sizes occasionally angular but 35766177 110599 51125 Page 281 Text v 30 http //www biodiversitylibrary org/page/35766177 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 35766177 dc identifier http //biodiversitylibrary org/page/35766177 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 N02/12734285014 2015-08-26 19 54 10 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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