148 MR J. D. FALCONER ON THE 



the silica. The alumina alone remains fairly constant. The silica percentage decreases 

 also towards the margins of the sills (III., IV.). This agrees very well with the results 

 of other observers, although Stecher's figures for the Hound Pt. diabase led him to 

 exact! v the opposite conclusion.* The segregation veins are exceptionally rich in silica 

 and alkalies. 



Conclusion. 



Three hypotheses have been advanced in explanation of the presence of an excess of 

 silica in some diabasic rocks. The quartz of the Scottish quartz-bearing diabases was 

 considered by Stecher entirely of foreign origin, and due to the fusion of portions of 

 the adjacent sedimentary rocks caught up during intrusion and the assimilation of their 

 constituents by the basic magma. t Later, the quartz was, on the analogy of quartz 

 syenites and quartz-diorites, conceived as an original constituent of the intrusive 

 magma ; J while Sollas has explained its presence on the assumption of the injection of 

 granophyric material into a previously solidified and porous diabase. § 



Stecher's idea undoubtedly contains some element of truth, for xenocrystic quartz 

 grains are frequently present on the margins of the sills. That the whole of the quartz- 

 content can be of this origin is, however, very doubtful. The remains of portions of 

 sandstones, shales, and limestones, which one would here expect to find, are rarely or 

 never observed within the diabases, while the neighbouring basaltic intrusions which 

 ought equally to possess such inclusions, are not only devoid of them, but of a micro- 

 pegmatitic matrix as well. 



Diabases with micropegmatite are a well-defined group of rocks, which occur in 

 various parts of the world, intruded into rocks of very different character. Their 

 constancy in composition and structure speaks more in favour of the primary igneous 

 origin of all their constituents than of the later and more or less accidental origin of 

 some of their ingredients. This remark applies also to Sollas' injection- theory, which, 

 though demonstrated in some cases as a junction-phenomenon, is very difficult to 

 apply to the case of the Linlithgow diabases. In the centres of the larger sills the 

 micropegmatitic matrix in places makes up about two-thirds of the whole, and in some 

 sections the columnar augites and felspars appear like porphyritic crystals in a grano- 

 phyric groundmass. It is improbable that the interiors of the Carribber and Kettlestoun 

 sills were ever occupied by such an exceedingly loose and open framework of augite and 

 felspar as the injection-theory would here demand. Moreover, the segregation veins, so 

 far as observed, are never themselves micropegmatitic, although formed out of the same 

 constituents as the intersertal matrix. 



On the whole, the facts are most easily explained on the hypothesis of the 

 differentiation of a diabasic magma primarily charged with an excess of silica. The 



* Stecher, op. cit. p. 161. t Tsch. Min. u. Petr. Mitth., vol. ix., 1887. 



X Teall, Quart. Journ. Geol. Soc, vol. xl., 1884, pp. 209, 640 : Harker, Ibid., vol. I., 1894, p. 311 ; vol. li. 

 p. 125: Holland, Ibid., vol. liii., 1897, p. 405. 



§ Trans. Roy. Irish Acad., vol. xxx., 1894, p. 477. 



