186 



I. Basalt-glass of the Beal, Portree. 



II. Basalt from the centre of the Lamlash dyke (DELESSE. Ann. d. Mines, 1851.) 

 III. Basalt-glass from the side of the dyke (DELESSE, loc. cit.) 



DELESSE'S analysis of the Lamlash dyke shows a higher silica per- 

 centage than is usual in basic eruptive rocks. 



Porphyritic dolerites and basalts occur in the Tertiary volcanic districts 

 of the West of Scotland. Felspar is the most important mineral which 

 determines the porphyritic character. It occurs in broad platy crystals 

 and not unfrequently shows a zonal structure. Augite seldom occurs in 

 porphyritic crystals. Rocks of basaltic texture are often rendered porphy- 

 ritic by the occurrence of olivine. Good examples of porphyritic dolerites 

 occur at Dun-da-gu in the Isle of Mull, in the peninsula of Ardnamurchan, 

 and at the Carlingford mountain in Ireland. On the north-east side of Mull, 

 between Tobermory and Ardnacroish, Prof. JUDD found a basalt containing 

 felspar crystals measuring more than two inches across. The Tertiary por- 

 phyritic dolerites resemble many of the rocks which have received such 

 names as labradorite-porphyry and diabase-porphyrite, both in structure 

 and composition. By a disappearance of the olivine and a reduction in the 

 amount of the ferro-magnesian constituents they pass into rocks of inter- 

 mediate composition. At Fair Head, Co. Antrim, occurs a remarkable 

 variety of porphyritic dolerite. The ground-mass of the rock is ophitic. 

 The porphyritic elements, instead of being individual crystals, are aggregates 

 of anorthite and olivine which range from a tenth to one-third of an inch 

 in diameter. The mutual relations of the anorthite and olivine are the 

 same as those which characterize troctolite or forellenstein. Prof. JUDD has 

 proposed that the term glomero-porphyritic should be used to designate the 

 structure of this rock. 



The Continental basalts of Tertiary age often contain nodules of 

 peridotite. These nodules may consist of olivine only, or of olivine in 

 association with enstatite, chrome-diopside and picotite. They may 

 measure only a fraction of an inch or several inches in diameter. It is 

 worthy of note that no such nodules have ever been detected amongst the 

 Tertiary basalts of Great Britain and Ireland. 



The only kinds of change to which the rocks now under consideration 

 have been subjected are those due to surface agencies, and possibly to 

 steam and other gases originating in connection with the final stages of 

 volcanic action in the district. There is no evidence of the alteration of 

 extensive masses into diabases, melaphyres, and wackes by surface agencies, 

 or of the development of schistosity, with its corresponding mineral 

 changes, by dynamic metamorphism. Nevertheless, we do find here and 

 there illustrations of the development of chlorite by the alteration of 

 augite ; of serpentine by the alteration of olivine ; and of kaolin and other 

 products by the alteration of felspar. Zeolites have also been extensively 

 developed. The alteration of augite into vermicular aggregates of chlorite, 

 similar to those represented in Fig. 2, Plate XXII., is well seen in the case 

 of a dolerite from Ardtornish. 



Midland Valley of Scotland. This district was the site of intense 

 volcanic activity during Old Red Sandstone times, when immense masses 



