OU EE EE 
426 PROF. T. G. BONNEY ON THE MICROSCOPIC STRUCTURE 
that this was from a dyke, and that the hornblende was a secondary 
formation from augite, so that the rock had once been a basalt. 
18 (Dyke Junction, Delancy Quarry).—Appears to be a junction 
specimen, a rock somewhat of the latter character cutting one of the 
former, both being a good deal altered. A vein of chalcedonic 
quartz cuts the finer rock. It is very possible that the coarse rock 
was once a gabbro and the other a basalt. 
44 (Quarry south of Baubigny Mill).—This rock is coarsely 
crystalline, and consists of felspar, agreeing best with labradorite, 
hornblende, and a little brown mica, apparently rather altered, a 
little apatite and magnetite (?); the hornblende appears to have 
crystallized later than the felspar, and may perhaps have been pro- 
duced by paramorphic or pseudomorphic action from a pyroxenic 
constituent. 
Mica-TRAP. 
The two specimens which may be included under this general 
designation come from the south-east of the island, one from 
Moulin Huet on the south, and the other from Bec du Nez on the 
east coast. The former (12) has a glassy-looking ground-mass © 
of a very pale brown tint, which is thickly studded with crystals 
of brown mica, many of them not exceeding about 0°001" in dia- 
meter, while others are as much as 0°03”, the latter including por- 
tions of the ground-mass, flakes of an almost opaque iron-mica, and 
microliths of apatite (?), together with a considerable amount of a 
mineral presently to be described. The ground-mass shows a 
rather indistinct trachytic structure, apparently consisting of a mass 
of elongated felspar microliths. The mineral mentioned above is at 
present evidently a secondary product. It is colourless, and occurs in 
prism-like flakes, not unlike one of the white micas which give 
moderately bright colours and have a silvery look with crossing 
nicols. Extinction takes place when the longer edges of the flakes 
are parallel with the vibration-planes of the crossed nicols, and, on 
the whole, I think that the mineral is probably tale. It occurs 
partly scattered in the slide, but also aggregated in more or less 
definite crystalline forms, which are outlined fairly continuously 
by granules of opacite. Within these it has a tufted habit, and is 
associated with a little ferrite. Exact parallels may be found in the 
excellent figures of rocks akin to mica-traps in the well-known work 
of Fougué and Levy, or in Professor Barrois’s recent volume *, where 
both augite and hornblende are figured as associated with mica, and 
these bear a general resemblance to our mineral. I may, however, 
note that in the former book, plate xxvi. (Porphyrite andésitique 
micacée, &c.) and plate xxvii. (Porphyre syénitique micacée, &e.) there 
is a mineral (replaced by serpentinous products) which in external 
form much resembles the above. The authors, with a little hesitation, 
ascribe it to bastite. That in the Moulin-Huet rock we have a 
magnesian bisilicate there can be little doubt. I should place it in 
the group for which I proposed the name of kersantite-porphyrite. 
The specimen from Bec du Nez (29) has a more definitely erys- 
* Recherches sur les Terrains Anciens des Asturies et de la Galice, pl. 1 & 2. 
