166 Prof. J. Miine—Distribution of Volcanos. 
the microscope fails to establish, but it is probably a mica. Unlike 
its fellows in the gneisses, it appears to form a nearly omnipresent 
material between individual quartz grains, and is not separated into 
distinctive bands. Hence its eminently fissile character.—T.D.|] 
[Nore 16.—Quartz in grains separated into nests and bands by 
very numerous wavy bands of muscovite. This constituent is also 
frequent as very minute lamine between individual grains, as in No. 
15, but is not so abundant as in that rock. The lamination charac- 
terizing these rocks is chiefly mechanical, and differs entirely from 
the banding of the gneiss.—T.D. ] 
[Nore 17.—An exceedingly compact bluish-grey rock, with dull 
fracture, soft, and resembling some indurated shales. Thickly im- 
pregnated with exceedingly minute crystals of iron pyrites. It is 
difficult to obtain a section of this sufficiently thin for examination 
with the microscope. The ground-mass, grey in colour, is feebly 
translucent, and incloses much quartz in minute grains.—T.D. ] 
[ Nore 18.—This rock has considerable resemblance to No. 17, but 
is characterized by its greater abundance of angular, subangular and 
rounded quartz grains, which frequently separate out into lenticular 
nests. A little mica in exceedingly minute crystals is present. A 
schistose structure is given to it by the tendency of the indefinable 
ground-mass to assume the wavy condition which characterizes the 
micaceous constituent of the schistose quartzites.—T.D. | 
[Norz 19.—This appears to be a micaceous quartzite. It consists 
principally of closely aggregated quartz-grains, the outlines of 
which are distinctly shown between crossed Nicols. There are two 
micas, but they are not abundant. The one, markedly dichroic, is 
irregularly distributed in minute crystalline lamine; the other, 
monochromatic, is in wavy continuous bands, breaking intermittently 
into lenticular groupings. The rock is traversed by veins of quartz, 
which is not distinguishable from that of the mass, but it contains 
no mica. A few fragments of felspar and rounded garnets are pre- 
sent, and also some crystalline nests of quartz. The detrital 
character of this rock is withal most marked.—-T.D. ] 
[ Norm 20.—The macroscopical and microscopical characters of this 
rock are the same with those of No. 15, with the exception that here 
the indefinite micaceous mineral is somewhat more abundant.—T.D. |] 
(Zo be concluded in our next Number.) 
V.—NoTE ON THE GEOGRAPHICAL DISTRIBUTION OF VOLCANOS. 
By Prof. J. Mitnz, F.G.S., 
Imperial College of Engineering, Tokei, Yedo. 
Tee what we know about the cooling of the earth and its 
geological structure, it would seem very probable that the 
principal features which we now see upon the surface of our planet, 
as, for instance, the continents and ocean-beds, received their forms 
in very early times, being, in fact, more or less a primary result of 
contraction. Since the formation of these impressions, contractions 
have continued to take place, and secondary results have come about, 
