GABBKOS ETC. IN SCOTLAND AND IRELAND. 59 



interrupted by the ejection of streams of augite-andesite lava of a pecu- 

 liar type ; but in both these districts such lavas are of comparatively 

 rare and even exceptional occurrence. There is, in fact, a remarkable 

 uniformity in the chemical composition of the basic rocks of the whole 

 district, from Iceland in the north to Ireland in the south. But 

 while some of the rocks are remarkably fresh and unaltered, others 

 are converted into amygdaloidal varieties, some of which are nndis- 

 tinguishable from the so-called melaphyres, basaltites, and palatinites 

 of the older geological periods. Nowhere do we find more striking 

 evidence than in the Scottish Isles that the degree of alteration in 

 an igneous rock is not an infallible criterion of its geological age. 



It may be interesting to recall the circumstance that certain 

 rocks, once confounded with the basalts of this district, were for 

 a long time adduced as affording the most striking evidence of 

 the truth of the AYernerian doctrine of the origin of basalt by 

 '^ aqueous precipitation." It is almost needless to remark that the 

 so-called " basalt containing ^m))?ou{^^s " of Portrush, Co. Antrim, is 

 nothing but a Lias shale altered by contact with the dolerites and 

 basalts, which have been intruded into it. The temporary interest 

 which was aroused with respect to these rocks, when the fossils were 

 supposed to be actually imbedded in the basalts, has, of course, alto- 

 gether subsided, now that the true relations of the igneous and 

 aqueous masses are better understood. 



In the Tertiary basalts of the British area, as in those of Iceland, 

 we find every variety, from forms which are nearly holocrystalline 

 and contain only a minute proportion of glassy residuum between 

 the crystals, to others in which the great mass of the rock exists as 

 a ground-mass or imperfectly crystallized base (magma-basalts). In 

 both areas tachylytes or perfectly vitreous forms of the basalt are 

 found, which are sometimes porphyritic, but they appear to be only 

 rare and exceptional occurrences, which can in every case be accounted 

 for by the abnormally rapid rate of the cooling of the rocks*. 



But when we turn to the consideration of the holocrystalline types 

 of these rocks, greater diversity of opinion as to their proper nomen- 

 clature is found to exist. As I have shown in my previous paper f, 

 the misconception as to the true nature of the pyroxenic constituent 

 of the rocks has led to much of the confusion ; but in 1871 Professor 

 Zirkel showed that the pyroxene in question is undoubtedly diallage, 

 and that the rocks present all the characters which petrographcrs agree 

 to be distinctive of the " olivine-gabbros." Professor Zirkel made a 

 careful microscopical study of a series of specimens from Mull, and of 

 another series from Skye. He showed that in both districts the rocks 

 are made up of perfectly crystallized plagioclase felspar, diallage, and 

 olivine, the whole of the minerals abounding with fluid-cavities. 

 He further pointed out tliat the olivine of these rocks exhibits the 

 characteristic structure of the olivine of gabbros, a structure which 

 he had never found in that mineral as it occurs in basalts. 



Professor Zirkel insisted, and rightly insisted, that as these rocks 



* See Quart. Jourii, Geol. Soc. vol. xxxix. (1883) pp. 444-465. 

 t Ibid. vol. xli. (1885) p. 357. 



