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In geology, just as in biology, there are two ways of studying
structure,—the small way and the large way. In the case of an organism,
we may select a single part or organ, and, disregarding its external
form and relations to other parts, observe its composition and minute
structure, the various forms and arrangements of the cells, etc. This is
histology, and it is the complement of that larger method of studying
structure which is ordinarily understood by anatomy.
The divisions of structural geology corresponding to histology and
anatomy are _lithology_ and _petrology_. Lithology is an in-door
science; we use the microscope largely, and work with hand specimens or
thin sections of the rocks, observing the composition and those small
structural features which go under the general name of texture.
In petrology, on the other hand, we consider the larger kinds of
rock-structure, such as stratification, jointing, folds, faults,
cleavage, etc.; and it is essentially an out-door science, since to
study it to the best advantage we must have, not hand specimens, but
ledges, cliffs, railway-cuttings, gorges, and mountains.