1889.] Mineralogy afid Petrograpliy. 719 
Judd.* It will be remembered that Judd,« in 1874, published an 
article in which was shown that there exist in the Western Isles of 
Scotland granites and gabbros, grading imperceptibly into vitreous 
pitchstones and tachylites, and emanating from five distinct centres, 
which were regarded as the seats of old volcanoes. The acid rocks of 
the region were regarded as older than the basic ones, and the district 
was thought to have been one of great volcanoes. In Dr. Geikie's 
monograph the last two conclusions are denied acceptance, while on 
the whole the first two are accepted. In his present article Judd gives 
his reasons for insisting upon the truth of his previous statements, and 
refuses to accept the view of Geikie that the great outbursts of lava 
took place from fissures (as in the Western States), rather than from 
volcanic vents. According to Judd the gabbros are the deep-seated 
portions of a magma, which, upon the surface, assumed the structure 
of basalt. This gabbro filled the fissures which were opened during 
the extravasation of the basalt, and is therefore contemporaneous with 
this rock. Geikie asserts that the gabbro injections belong to a dis- 
tinct and later period than the outflow of the basalt. The controversy 
bids fair to yield results of great interest to petrography ; for this 
reason it has been referred to in this place. The acid rocks of this 
region present some very interesting appearances, which are described 
by Judd® in a short paper. Granitic eruptive masses usually pass 
towards their peripheral portions into granophyres, smaller eruptive 
bosses and laccolites exhibit the granophyric structure throughout; 
while apophyses from intrusive masses display the same structures, 
sometimes on a very minute scale. A labradorite-andesite is composed 
of large crystals of labradorite scattered through a glassy base con- 
taining microlites of feldspar, augite and magnetite. In a specimen 
of this rock from Dun da Ghaoithe in Mull are large idiomorphic 
labradorite crystals, consisting of a central, sometimes rounded and 
corroded, core surrounded by an irregular fringe of the same mineral 
substance,^ differing from the core in extinction and in other prop- 
erties. This enlargement takes place only where the original crystal 
was in contact with the glassy matrix. The crystallographic con- 
tinuity of the core and the surrounding envelope is shown by the 
passage of twinning planes from the one into the other ; the optical 
* The Tertiary Volcanoes of the Western Isles of Scotland. Quart. Jour. Geo. Sac, 
May, 1889. PP- 187-219. 
