362 rPvOF. J. ^y. judd on the peoptlites 



as I shall show in a subsequent part of this paper, are so clearly 

 with the vitrophyric pyroxene-andesites, that it is quite impossible 

 to remove them from the group of the andesites. We must regard 

 them therefore as the most basic type of the andesite series, — forms 

 which constitute a real connecting-link between the andesites and 

 the true or oli vine-basalts. I am still inclined to follow Zirkel's 

 original rule of confining the name basalt to those rocks of which 

 olivine forms are essential constituents. This plan is followed by 

 Rosenbusch in the case of the leucite and nepheline-bearing rocks, 

 though of late years he has departed from it in the case of the 

 felspar-bearing types. 



Augite-andesites of the vitrophyric type of Eosenbusch appear at 

 many points in the Western Isles of Scotland, and are particularly 

 abundant about the north-western part of Beinn-a-Ghraag in Mull. 



In these we find crystals of augite and enstatite, of a felspar 

 allied to labradorite, and of magnetite, embedded in a glassy ground- 

 mass, which may be large or small in quantity compared with the 

 crystalline constituents of the mass. As in almost all similar rocks, 

 we may notice that the crystals are often by no means uniformly 

 scattered through the glassy base, but are collected into groups 

 which often appear like portions of a holocrystalline mass. 



At Mhaim Clackaig in Mull I have found a vitrophyric augite- 

 andesite in which crystals of labradorite (usually much rounded on 

 the angles and sometimes corroded), of a brown augite, and of mag- 

 netite are somewhat sparsely scattered through a glassy base. This 

 glassy base is crowded with black rods (trichites), much twisted 

 and bent, which in places become so abundant as to render the 

 glass nearly opaque, except in very thin sections. This rock has a 

 specific gravity of 2-64 (see Plate XIV. fig. 3). 



In Beinn-a-Ghraag similar glassy rocks are highly spherulitic, the 

 spherulites being arranged in definite bands, evidently produced 

 during the movement of the viscous mass (see Plate XIV. fig. 4). 

 One of these spherulitic rocks has a specific gravity of 2*49. In 

 other cases, the fluidal structure, indicated by the way the micro- 

 lites of the second period of consolidation are arranged around the 

 porphyritic felspar and pyroxene crystals (see Plate XIV. fig. 6), is 

 very strikingly shown. 



Por the determination of the specific gravities of a series of the 

 vitrojjhyric augite-andesites I am indebted to my assistant, Mr. F. 

 H. Hume, P.G.S. 



Although no augite-andesites with free quartz have been detected 

 in the district, yet some of these very glassy varieties must have a 

 silica-percentage as high as that of the quartz augite-andesites or 

 augite-dacites. 



The best type of the more basic, stony pyroxene-andesites (" tra- 

 chytoid andesites " of Eosenbusch) is afforded by the rocks which 

 exactty resemble the lavas of Stikkisholmur and other points in 

 Iceland, and are so well described and figured by Breon*. The rock 



* Loc. cU. pp. 23 & 24, pi. iii. fig. 1. 



