132 Transactions of the Royal Microscopical Society. 
Application of the above-described Principles to Special Cases. 
The practical application of the general principles already 
explained will, I think, be more readily understood if I describe a 
few characteristic examples of various uatural deposits. 
Millstone Grit of South Yorkshire.—It would be difficult to 
find a better example of a coarse-grained sandstone, almost entirely 
derived from granite, than the above-named rock. Some of the 
beds can easily be broken up into loose sand, and the structure 
of the grains observed when mounted in balsam. With very 
few exceptions they are extremely angular, as shown by Fig. 13, 
and in every respect identical with grains of quartz derived from 
decomposed granite. Grains of unchanged felspar do sometimes 
occur, but the greater part has been decomposed into clay, which 
has been squeezed into the spaces between the grains of quartz. 
In some few cases portions of felspar may still be seen adhering 
to quartz, as shown by Fig. 14, which may therefore be said 
to be actual grains of granite. The quartz is on the whole very 
free from fluid cavities, and more like that from the granites of 
Norway than from any British variety which 1 have examined, 
and certainly very unlike that from the Cornish granites. These 
conclusions agree admirably with other facts. In the associated 
pebble beds portions of undoubted granite may be found. It is 
of coarse grain, with comparatively clear felspar, quite unlike the 
usual varieties met with in Scotland or Cornwall, but closely like 
those from Norway. ‘The current structures of the rock clearly 
show that the material was drifted from the north-east, and it was 
probably derived from granitic rocks lying at no great distance in 
that direction. Even the much finer grained quartzose sand beds, 
like the gaunister, have apparently been mainly derived from 
granitic rocks. We need not go far.to find the other constituents 
of the granite. Mica abounds in some beds, and the decomposed 
felspar has no doubt largely contributed to the material of the 
associated shales and indurated clays. Nearly all the grains of 
quartz are as angular as if never subjected to attrition ; but a few 
are so much worn that they may have had a different history— 
they may have been exposed longer to wear, or may have been de- 
rived from dunes of blown sands. 
Sand of the Egyptian Desert,—This is a splendid example 
of a sand which has been very much worn by attrition, as shown 
by Fig. 15, all angles being removed. Ordinary dune sand shows 
the same kind of wearing in a less degree. When blown about 
by the wind the friction of the grains one on another is necessarily 
much greater than when they are to a considerable extent buoyed 
up in water, and there is thus no difficulty in understanding 
