Prof. Hull — On the Structure of Irish Granites. 195 



the cross-banded structure is characteristic of granitic rocks. Dr. 

 Zirkel has, I believe, observed a similar structure. 



This structure differs from that of the triclinic felspars, in that 

 the bands of colour are somewhat irregular, interrupted, and crossed 

 by other bands perpendicular, or nearly so, to the former. On 

 rotating the analyzer, the colours change through purple shading 

 into yellow ; grey, and blue shading into green. At first I was in 

 doubt regarding the cause of this peculiar appearance ; but with a 

 high magnifying power all doubt vanished. It was then seen to be 

 due to a cellular, or tubular, structure ; the whole mass of the 

 mineral being penetrated in two directions by a series of irregularly 

 parallel tubes, or in reality planes of cells, tapering in both directions 

 and appearing like strings of little beads hanging from invisible 

 points. This structure is be§t seen by using the lower prism 

 ("polarizer") only, and observing the section with the j -inch object- 

 glass, when it will appear somewhat like that represented in Fig. 2. 

 Some of the lines of cells are straight, others bent a little, while the 

 intervening portions of the mineral are perforated by very minute 

 solitary cells. A portion of one of the longitudinal bands of cells is 

 represented in the upper part of the figure. These are visible 

 with the :|^-inch power in special parts of the thin slice, and though 

 not so numerous as the transverse bands, are more continuous. They 

 are also lines or bands of weakness ; for in the slice, the margin of 

 which passes across one of those cross-banded felspar crystals, there 

 is always a little notch at the point where the band meets the edge. 

 In speaking of this cross-banded felspar as " orthoclase," I am 

 assuming that the large crystals in the slice belong to the large 

 flesh-coloured crystals in the rock itself, which are known, by the 

 determination of Dr. Haughton, F.E.S., to be orthoclase. Of this I 

 have no doubt ; and even if the evidence was less positive under this 

 head, I should have concluded the mineral to be orthoclase from the 

 absence of the fine parallel lines and' bands of colour when viewed 

 with polarized light. As regards the arrangement of the longer (or 

 horizontal) bands of cells, my friend Dr. Emerson Eeynolds, on 

 viewing the slice, suggested to me that they must coincide with the 

 planes of cleavage of the crystal, as it would be naturally along such 

 planes that the cells, whether filled originally by gas or steam, would 

 arrange themselves. The parallelism of these bands confirms this 

 view. 



Mica. — The foliated structure of the granite is well exemplified 

 by the arrangement of the mica in the slice. Besides scattered 

 flakes imbedded both in the silica and felspar, there are three irre- 

 gularly parallel bands crossing the section transversely, and com- 

 posed of numerous sets of twisted flakes, sometimes rich brown or 

 bronze in colour, sometimes almost colourless. As usual, they are 

 all deeply scarred, and are sometimes quite opaque, or slightly trans- 

 lucent. One minute ci-ystal, showing the hexagonal form, is inclosed 

 in a cavity in felspar (see Fig. 3). Some of the black specks seen 

 in the mica I suspect to be magnetite, representing the excess of iron 

 over that which has entered into the composition of the mica itself. 



