Anniversary Address of the President, H.C. Sorby, F.R.S. 115 
In examining each particular deposit two different questions 
present themselves. It is necessary, in the first instance, to 
identify as accurately as possible the mineral nature of the various 
large or smaller particles, and in the second place to determine as 
far as is possible the true nature of the rock from which they were 
originally derived. Thus, for example, if we were studying some 
modern sandy mud, it would be necessary to identify the grains of 
quartz, felspar, mica, and hornblende, and the minute granules 
derived from more or less completely decomposed felspar; but 
after this preliminary step another important question presents 
itself. It is very desirable to determine the nature of the pre- 
viously existing rocks, which, when decomposed and broken up by 
various chemical and mechanical actions, gave rise to the particles 
of the mud under examination. When this can be successfully 
accomplished, the history of a comparatively modern deposit may 
be indefinitely extended into remote past epochs. In a similar 
manner the study of the ultimate constituents of the very oldest 
stratified rocks might enable us to form some opinion respecting 
the nature of still earlier rocks, of which no other record may 
remain. If this could be done successfully we might, as it were, 
sometimes trace back the genealogy of our globe a generation or 
more earlier than by other means. This appears to me a question 
of so much interest, and its solution so dependent on microscopical 
investigations, that I venture to bring it before you in some detail, 
even although the conclusions have a more direct bearing on 
geology than on those branches of science which usually claim the 
attention of this Society. 
Preparation of the Objects. 
When stratified rocks are sufficiently hard and consolidated to 
be made into thin and partially transparent sections, many facts 
may be better seen in slices cut perpendicular to the stratification 
than by attempting to disintegrate the rock and examine the 
detached particles. It would, however, often be difficult or almost 
impossible to prepare satisfactory thin sections of many modern or 
ancient deposits, and it thus becomes necessary to study them in 
another manner. If the particles are firmly held together by 
calcic or ferrous carbonate, or by any of the oxides of iron, they 
may be set free by the action of cold dilute hydrochloric acid, or by 
a stronger hot solution; but if, as often happens, the rock is con- 
solidated by means of silex, this cannot be accomplished. Violent 
mechanical crushing must be avoided, since it would give rise to 
false results by fracturing the constituent grains. Such an amount 
of crushing as can be effected with a small stiff brush made with 
bristles does, however, appear to be admissible, since it could 
scarcely break the separate fragments. Even in the case of 
K 2 
