134 Transactions of the Royal Microscopical Society. 
which decompose into similar material. Taking all these facts 
into consideration, there can be no reasonable doubt that a very 
large portion of these rocks has been derived from schists, which 
has been deposited along with a little mica and many very fine 
granules, derived from perhaps more distant granitic rocks. 
lt appears to me scarcely necessary to describe any other 
special cases. As far as I have been able to ascertain from an 
examination of sandy deposits belonging to nearly every period of 
our British stratified rocks, I think we may conclude that, as a 
general rule, the coarser-grained sands are mainly derived from 
granitic, and the finer from schistose rocks; which is no doubt 
because the separate solid fragments of quartz in granite are 
usually much larger than those in schists. Even the oldest slates 
which I have examined thus furnish evidence of the existence of 
still earlier strata, subsequently metamorphosed. One thing which 
may appear somewhat surprising is that on the whole the grains of 
sand are very little worn, and hence the finer-grained sands have 
not resulted from the wearing down of larger grains, but consist of 
particles which were originally small, separated from the larger by 
the action of currents. That part of the quartz worn off from the 
rounded grains is probably met with in the clays mixed with 
the true kaolin. It seems scarcely possible that the larger solid 
grains could be reduced to smaller by simple fracture, by the action 
of currents. 
The microscopical structure of the quartz enables us to form 
some idea of the general character of the granitic rocks which have 
yielded so large a part of the coarser sands met with in British 
strata. They must certainly have been very unlike the Cornish 
granites, since the quartz of these latter contains a far greater 
number of fluid cavities. They must have been much more like the 
granites of the Scotch Highlands or those of Norway, and perhaps 
we should not be far wrong if we were to conclude that they be- 
longed to a type intermediate between these, which formerly 
occurred in an area now no longer dry land. 
The difference in the colour of different sands depends on the 
amount and condition of the oxide of iron, forming as it were a 
superficial varnish. This is easily seen when the grains are 
mounted in balsam. ‘The quartz itself is the same, and since the 
state of the oxide might be so soon changed, there is no difficulty 
in understanding why sands of such very different colours may be 
associated together. 
The green grains of glauconite cannot as a general rule have 
been derived from pre-existing rocks, and ought rather to be 
attributed to chemical action occurring either during or soon after 
deposition, like the minute crystals of gypsum met with in some of 
the dredgings from the Pacific Ocean. 
