Anniversary Address of the President, H. C. Sorby, F.R.S. 119 
does not enable us to exceed this limit, for reasons already 
explained. The value of the larger aperture in studying an 
entirely different kind of object is another question altogether. 
On the Microscopical Character of Sands and Clays. 
In studying loose and unconsolidated sands and clays little or 
nothing can be learned respecting the structural arrangement of 
the particles. Our attention must be almost entirely confined to 
their mineral nature, external form, and internal structure. The 
determination of some facts is certainly facilitated by our being 
able to study detached particles ; but the observation of other facts 
is made more difficult by our not being able to examine transverse 
sections of the individual grains. In any case we are obliged to 
make use of materially different methods of study, and must rely 
on different classes of facts. I feel very strongly how imperfect 
the whole subject still is. I have had great difficulty in obtaining 
suitable material, since, as a rule, such specimens as are of the 
greatest interest in connection with this investigation are seldom 
collected, and I have been obliged to rely almost entirely on what I 
collected for myself some years ago for this special purpose. 
Origin of the Material. 
In studying any particular stratified rock it would usually 
much assist us in forming a true opinion respecting its formation, 
if we could ascertain the previous history of the material of which 
it is composed. Thus in the case of limestones it is desirable that 
we should know the exact nature of the calcareous organisms, 
which, by being broken up or decayed, yielded calcareous sands or 
muds, since consolidated into hard rock. Very often this may to 
a great extent be learned from the organic structure characteristic 
of various groups of shells or corals. In some of our stratified 
rocks we meet with larger or smaller fragments of previously 
existing rocks, and even some of the oldest slates in Wales are seen 
with the microscope to be, as it were, museums of specimens of the 
rocks existing at the time of their deposition. The further study 
of this branch of my subject may probably enable us to prove that 
these ancient Welsh slates were in part derived from previously 
existing strata, which were themselves derived from still earlier 
rocks; but time will not allow of my entering into this question. 
I purpose to limit myself to those facts which show the nature of 
the rocks which, when decomposed and broken up, yielded the 
detached grains of sand or other mineral particles subsequently de- 
posited and consolidated. So very little attention has been paid to 
