GABBKOS ETC. IN SCOTLAND AND IKELAND. 71 



Porphyritic basalts are extremely common in the area; indeed 

 they preponderate over non-porphyritic varieties ; in other Tvords the 

 majority of hasalts contain, scattered through their ground-mass, 

 certain crystals which are distinguished by their larger size, and 

 frequently by other characters also, from those forming the ground- 

 mass itself. The mineral which usually occurs in porphyritic 

 crystals is the felspar ; but olivine in large grains is sufficiently 

 frequent; while augite, so abundant as a porphyritic constituent 

 in the basalts of other petrographic provinces, seldom or never 

 occurs in that form in our rocks. 



At a point on the north-east side of Mull, between Tobermory 

 and Ardnacroish, I found a basalt containing porphyritic felspar 

 crystals which were no less than two inches across. The easy 

 cleavability of the porphyritic crystals, and the toughness of the base 

 in which they were imbedded, made it quite impossible, however, 

 to obtain really representative hand-specimens of the rock. 



Porphyritic Tachylytes. — In some of the basalt-glasses of this 

 district, as has been already pointed out *, porphyritic crystals, not 

 only of felspar and olivine but also of augite, occur not UDfrequently. 

 The corroded appearance of such crystals has been already described. 



4. The Glo7nero-porphyritic Structure. — In the remarkable ophitic 

 dolerite of Pair Head, Co. Antrim, we find an example of a structure 

 which, so far as I am aware, has not hitherto been described, and to 

 which the above name may be applied. The base is an ordinary ophitic 

 dolerite, but scattered through this base we find groups of anorthite- 

 and olivine-crystals which appear like scattered fragments of an 

 olivine-anorthite rock or troctolite (see Plate YII. fig. 3). The 

 relations of the anorthite and olivine in these inclusions, which vary 

 from one tenth to one third of an inch in diameter, is similar to 

 that which is so well known as characterizing the Forellenstein 

 or troctolite. The olivine is not altered to magnetite along the 

 cracks, as is so frequently the case in the troctolites, but the anorthite 

 contains a large number of secondary inclusions. I have not been 

 able to detect any clear indications of change along the edges of 

 these groups of crystals, where they are in contact with the enclos- 

 ing ophitic dolerite, but the felspars are certainly traversed by an 

 enormous number of fine cracks; this is the more striking inasmuch 

 as the associated olivines are almost unaltered, and do not send out 

 radiating processes of serpentine into the felspars, as is so frequently 

 the case with the troctolites in which the process of serpentinization 

 of the olivines has commenced (see Plate VII. fig. 3). 



The only approach to this glomero-porphyritic structure with 

 which I am acquainted is that exhibited by some dolerites — for 

 example, those of Carboniferous age described by Mr. Allport t, in 

 which we find radiating groups of felspar-crystals sometimes associ- 

 ated with large augites, both of these minerals showing some 

 characters which distinguish them from the mass of the crystals in 

 the rock. Those who regard the enclosures composed of olivine, 



* Quart. Journ. Geol. Soc. vol. xli, (1885) p. 444, plate xiii. 

 t Quart. Journ. Geol. Soc. vol. xxx. (1874) p. 529, &c. 



