126 Transactions of the Royal Microscopical Society. 
off, so as to make it consist of little more than a mass of irregular 
cells with thin dividing walls—a sort of glassy froth. As an 
example of a fragment, I refer to Fig. 12, in which the shaded 
parts represent the thin cell-walls seen edgewise. 
Tron Oxides.—As a general rule the oxide of iron derived from 
the decomposition of rocks is the hydrous peroxide. Sometimes the 
separate granules can be recognized, but it often occurs as a mere 
staining over the surface or in the interior of the larger fragments, 
made up of granules too small to be separately defined. We thus 
obtain various tints of red and yellow, made more or less brown by 
the presence of the magnetic oxide. In some cases we also have 
larger grains derived from crystals of this latter mineral which 
were present in the original rock previous to its decomposition. 
Changes taking place after deposition may soon alter the amount 
of combined water, or give rise to ferrous compounds, or to pyrites, 
and completely alter the colour of the rock, and may thus greatly 
obscure the relation between modern and more ancient deposits. 
General Conclusions. 
Taking into consideration all the facts described above, I think 
we are able to draw the following general conclusions. On the 
whole, the particles derived from the decomposition and breaking up 
of granitic rocks are sufficiently different from those derived from 
schists to make it possible to decide to which of the more usual types 
of those two classes of rocks they should be referred. When we 
come to study individual particles, many may be found which show 
the characteristic differences so very imperfectly that it may be 
impossible to determine their origin in a satisfactory manner. In 
examining any particular deposit, we must consider whether the 
relative amount of such doubtful cases is greater than would 
correspond with the amount of those clearly indicating that they 
were derived from granite or from schists. Thus, for example, if 
the great bulk of the material is like particles derived from schists, 
the presence of a few grains of quartz which might have been 
derived from granite may safely be attributed to the thicker folia 
or quartzose aggregations in the schists ; whereas, if the great bulk 
is made up of particles like those derived from granite, the pre- 
sence of a few doubtful grains may be ascribed to exceptional 
variations. We must, however, of course be prepared to find that 
many stratified rocks have been formed from material derived from 
both sources ; and, on the whole, the only safe conclusion is to say 
that they have been mainly derived from one or the other, or from 
both in some simple proportion, which can be only approximately 
known. 
