134 
Chapter IV. 
the beacon sandstone and other sedimentary rocks. 
The Beacon Sandstones are medium-grained rocks, for the most part remarkably 
free from ferruginous or other coloured impurities. The quartz-grains which compose 
them have doubtless been derived from the granitic and gneissic rocks, as they show 
liquid inclusions with bubbles, and occasionally numerous hair-like inclusions (rutile?) 
such as are often found in the quartz of granites. In some of the grains these 
needles occur in lines perfectly straight and parallel to the directions of extinction. 
Accessory constituents are rare in these sandstones, but in some specimens opaque and 
kaolinised white and pink felspars are plentiful. In 
a fine-grained quartz-grit (641) at the base of Finger 
Mountain are small irregular grains of pink garnet 
and rounded crystals of zircon and rutile, and in 
coarse-grained felspathic grits (640, 642) from the 
same locality occur large angular fragments of micro- 
cline. In these grits the quartz-grains are quite 
angular, but most of those in the ordinary sandstones 
are fairly well rounded. 
The cementing material is siliceous and usually 
is not in large amount, so that the grains are loosely 
cohering, but in the “ stalagmitic ” and “ marbled ” 
sandstones, mentioned on p. 44, narrow seams of the 
rock have been converted into compact quartzite, owing 
probably to local infiltrations of siliceous material. 
Under the microscope such parts of the sandstone 
(679) show rounded grains of quartz cemented by 
accretions of quartz in crystalline continuity with the grains, as in the case of the 
stiperstones of Shropshire. Between crossed nicols the slide has the appearance of 
interlocking irregular quartz-grains, but in ordinary light the perfectly rounded oval 
outlines of the original grains are clearly seen (see Fig. 71, in which the dotted lines 
show the outlines of the original rounded grains). 
Of the larger pebbles in the Beacon Sandstones, most consist of granitic quartz 
with liquid inclusions and moving bubbles, but one specimen (638) from below Finger 
Mountain appears to have been derived from an earlier sandstone, as it consists of 
rounded and sub-angular quartz-grains cemented by quartz. Another pebble (675) 
from the sandstone under B, consists of a quartz-schist showing, under the microscope, 
irregular interlocking grains of quartz, with strings of carbonaceous material marking 
Fig. 71 . — Quartz-grains in Beacon 
Sandstone ( 679 ) from Inland Forts. 
The dotted lines show the original 
rounded outlines of the grains. (Mag- 
nification, 20 diam.) 
