126 Prof. Bonney—Microscopic Structure of Pre-Cambrian Rocks. 
at the edges; though the latter are sometimes in part bounded by 
straight lines. In these, and occasionally in the ground-mass, are 
minute scales of a clear mica. The quartz is very clear. Magnetite 
(with some hematite) may be recognized. Part of the slide exhibits 
a brecciated structure, but after careful examination I am of opinion 
that this is due to brecciation in situ;! and that the rock is a quartz- 
felsite. 
(5) Differences only varietal, ground-mass less “ arborescent ” in 
structure, less porphyritic, no mica, but a minute granular pale-green 
(hornblendic ?) mineral. Less brecciated ; also a quartz-felsite. The 
structure suggests the possibility of its being a contemporaneous 
lava. (6) Structure of ground-mass yet more minute and granular ; 
parts of the slide show a distinctly clastic structure, which it is very 
difficult to suppose the result of brecciation im situ. In any case, I 
believe the rock the result of volcanic action; and think it most 
probably an altered trachytic tuff. (7) Ground-mass nearer to that 
of first two; but more minute in structure. Outlines of quartz and 
felspar more regular than in (4); former mineral less “clean.” 
Numerous almost colourless microliths in the ground-mass. Rather 
brecciated, but as the structure much more resembles that in (4), I 
think the rock a true quartz-felsite, possibly once a lava flow. 
(B.)—Granitoip AND Gnetssic Rocks (2, 8, 9). 
These three rocks consist mainly of quartz, felspar, and mica, and 
are very granitoid in aspect, exhibiting no distinct foliation. Still 
the outline and arrangement of the component minerals suggest that 
they are metamorphic rather than true igneous rocks. (2) Quartz 
rather full of minute cavities. Among the felspar, a considerable 
amount plagioclastic. Microliths frequent ; probably decomposition 
products ; green mica (biotite) anda little white mica. A granitoidite. 
(8) The specimen is not a favourable one for examination, being 
taken (probably unavoidably) from an old surface. At the first 
glance it seems indubitably clastic, but closer examination shows that 
much of this structure has been produced in situ; crystals of felspar 
being broken through and recemented by vein quartz, etc. ; so that 
we cannot cite it as a proof of clastic origin for the rock itself. Still 
so far as the true structure of this can be ascertained, it is that of 
a granitoidite, rather than a true granite. Some of the felspar is 
orthoclase ; there is also finely banded plagioclase, and perhaps a 
little microcline—a good amount of opacite and ferrite, and a little of 
a clear micaceous mineral, similar to that already described in rocks 
of this kind.? 
(C.)—Drorits. 
(3) In this specimen the felspar is a good deal decomposed ; the 
hornblende is green, showing good dichroism and characteristic 
cleavage, but is more or less irregular in external form. Ferrite, 
opacite, and larger grains of some iron oxide present. A trace of a 
* Produced by crushing or strain: not uncommon near faults. 
_* Quart. Journ. Geol. Soc. 1879, vol. xxxv. p. 322. 
