File:The Quarterly journal of the Geological Society of London (12893450595).jpg

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MR. W. P. D. STEBBING ON TWO BOULDERS OF .May 1 897,
grains of a mineral having a very slight earth -brown tint in
ordinary light, and appearing (with crossed nicols) as an aggregate
of minute specks inclining to flaky or fibrous, which bave weak
depolarizing power

probably a pinite (? after cordierite). A very
little of rather altered biotite and iron oxide, also zircon, and (?)
rutile, both small and rare. The structure of the rock is not that
of a normal granite, but more nearly resembles one of those which
are often banded, and commonly occur in a complex of the older
Archaean, such as the so-called Laurentian or Hebridean (see ng. 1).
'Boulder B. — The constituents in this specimen differ, in that much
of the felspar is ordinary plagioclase, and orthoclase is more certainly
present: the pinitic mineral is doubtfully present; there is very
Fig. 1. — Microscopic section of Boulder A.
X30.
The dark grain at the edge, about S.S.W. of the centre, is blackened biotite,
and there are some small films to the N.W. of it. The lighter grains are
quartz

the rest is felspar. A trace of minute micrographic structure is
visible. The rock is cracked, but the peculiarly formed clear space
between S.E. and E.S.E. is caused by a breakage in the slice.
little biotite, but much white mica. A little of this may be
bleached biotite, and in one or two cases I think that small flakes
of biotite are interlaminated. There is a small elongated cluster
of impure red garnets, pierced longitudinally by a thin tongue of
quartz (see fig. 2). In this rock signs of mechanical action are
clear. The mica is sometimes bent or 'rucked,' the quartz