1904 - 5 .] G-raytolite-bearing Bocks of the South Orkneys. 465 
the same as in the normal greywacke, i.e. pebbles of quartz and 
chalcedony, pieces of shale, small crystals of plagioclase, a few 
grains of sphene and zircon, and biotite flakes. Of the larger 
quartz pebbles, some at least are typical plutonic quartz, with lines 
of fluid inclusions, but showing strain shadows : the majority 
seem to be derived from some metamorphic rock — pebbles which 
in ordinary transmitted light appear quite uniform, between 
crossed Nicols are seen to be composed of a mosaic of different 
crystallographic individuals. The cementing material is not so 
obvious as one would expect from a naked-eye examination, as 
the interstices between the larger pebbles are filled up by smaller 
fragments, chiefly of quartz. It has the same chalcedonic 
appearance as in the typical greywacke, but green chloritic flakes 
are more abundant. There are numerous small veins of both 
calcite and quartz : one of the latter, about 0‘03mm. in width, was 
observed running right through some of the large quartz pebbles. 
2. Greywacke-slate. Has a fine laminar structure parallel to the 
places of deposition, is of a lighter grey colour, and splits up readily 
into thin laminae. There is no true slaty cleavage developed, 
however. 
3. Greywacke, showing gneissic banding and folding. This was 
only got in one patch of very limited extent. 
Shaly rocks also occur. In one situation only were regular beds 
of shale found alternating with layers of greywacke. Commonly 
the shale occurs simply as patches in the greywacke, seemingly 
irregularly mixed up with it, or with ill-defined borders shading 
off into the greywacke. The shale is much cleaved and broken, 
the individual pieces being bent and curved, and showing numerous 
slickensided faces, the result of the crushing and faulting to which 
it has been subjected. Microscopically it shows much brownish- 
grey amorphous material and black carbonaceous matter in the 
lines of stratification — forming a sort of network in the silica 
matrix. Interstratified lenticular-shaped patches occur, which are 
much freer from amorphous matter. With crossed Nicols these 
resolve themselves into a crypto-crystalline chalcedony, identical in 
character with the cementing material of the greywackes. 
The largest development of the shale occurs on a small islet off 
the south coast of Laurie Island, near Cape Dundas — its eastern 
PEOC. ROY. SOC. EDIN.— YOL. XXV. 30 
