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| Keywords: The Quarterly journal of the Geological Society of London (12733790295).jpg 280 J W JUDD ON THE SECONDARY ROCKS OF SCOTLAND <br> direction of the flow while the actual surfaces of the stream exhibit <br> the most strikingly scoriaceous aspect The same weathering pro- <br> cess sometimes develops in these old lavas other original structures <br> which had become wholly obliterated by infiltration crystallization <br> and other processes taking place in the mass of the rock ; and these <br> structures would have remained altogether unsuspected but for the <br> action of this cause Thus some highly crystalline and porphyritic <br> rocks when weathered resume their earthy or compact texture ; and <br> in certain cases structures like the ephaerulites of pearlstone which <br> had become wholly obscured in the mass are again revealed Simi- <br> larly many rocks of very solid appearance are seen when the infil- <br> trated materials are removed by weathering to have been originally <br> aggregates of ashy pumiceous and scoriaceous fragments among <br> which volcanic bombs or their fragments may not unfrequently <br> be detected <br> 3 Relations of the Volcanic Roclcs of Lorn ” In considering this <br> question it is important at the outset to notice two striking facts with <br> regard to the positions of the lavas and associated beds of Lorn In <br> the first place they always rest directly but unconformably upon <br> the Lower Silurian gneissose and schistose rocks and never exhibit <br> any of the fossiliferous Secondary strata at their base ; in this respect <br> they present a striking difference in their relations from the Terti- <br> ary lavas of the adjoining districts And secondly they are like <br> the older rocks upon which they repose penetrated in every direc- <br> tion by numerous dykes of dolerite and basalt ; these are precisely <br> similar to the intrusive masses associated with the Tertiary volcanic <br> rocks ” of which series indeed we can scarcely hesitate to regard <br> them as forming a part <br> These later dykes of dolerite and basalt which are sometimes of <br> great width and often present a prismatic or columnar structure con- <br> stitute a most interesting and striking feature in the district In <br> consequence of their usually greater relative hardness they frequently <br> stand up like gigantic walls amidst the rocks of slate and felstone <br> which once enclosed them but have now been weathered away from <br> their sides <br> The remarkable series of volcanic rocks of Lorn is probably several <br> thousand feet in thickness ” though its upper portions having been <br> everywhere removed by denudation its original limits in this respect <br> are quite unknown to us Its relations to the slate rocks and certain <br> conglomerates sandstones and breccias appear at first sight to be <br> That the geologist never meets with glassy lavas obsidians pitchstones c <br> among the older rocks will not occasion any surprise when we reflect upon the <br> facility with which artificial glasses undergo devitrification That glassy lavas <br> were formed even in palaeozoic times appears to be indicated by the fact recorded <br> in the text that weathering sometimes reveals the characteristic sphcerulitic struc- <br> ture in some of the lavas of Lorn Many of the Newer Palaeozoic porphyrites <br> of Scotland are quite undistinguishable in appearance from some trachytes such <br> as those of Hungary The white granular siliceous rocks into which the former <br> are sometimes found altered appear to be equally undistinguishable from the <br> products of the decomposition of the latter occasioned by the passage of acid <br> vapours through them and constituting the so-called Alaunstein ' 35766175 110599 51125 Page 279 Text v 30 http //www biodiversitylibrary org/page/35766175 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 35766175 dc identifier http //biodiversitylibrary org/page/35766175 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 N02/12733790295 2015-08-26 20 05 08 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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