
STRATIFICATION 105 
The structural geology of derivative rocks is upon the 
whole simpler and more readily understood than that of 
eruptive and metamorphic rocks, and, therefore, we shall 
consider first the phenomena which are specially characteristic 
of aqueous or sedimentary deposits. As we shall learn in 
the sequel, however, many of the structures presently to be 
described are met with likewise among igneous, and some of 
them even among metamorphic, rocks. 
Consolidation of Incoherent Accumulations.—By way 
of introduction to the present subject, a few remarks on the 
consolidation of rocks may not be out of place. Rocks, as 
we have learned, are not all equally compacted, and their 
state of solidification is no certain test of their relative 
age. It holds generally true, however, that the fragmental 
accumulations of early geological ages are more consolidated 
than those which have been formed in later times. We 
cannot doubt that conglomerates, sandstones, shales, lime- 
stones, tuffs, and volcanic agglomerates were formerly as 
loose and incoherent as any similar masses now in course of 
formation. There is one obvious way in which some of these 
accumulations have become hardened, and we can see the 
process in operation at the present day. Water percolating 
through loose sand and gravel introduces mineral matter 
which, as the water evaporates, is deposited between the 
grains and pebbles, and thus binds these together. Frequently 
a sediment becomes compacted by the chemical action of 
water upon its own constituents. Calcareous accumulations, 
for example, tend to become consolidated by the solution of 
the calcium-carbonate, and its subsequent precipitation in 
pores and interstices; what were formerly yielding incoherent 
masses becoming in this way converted into hard rocks, such 
as calcareous sandstones, grits, and limestones. We know 
also that loose or soft materials may be compacted by the 
weight of overlying masses. Peat, for example, taken from 
the bottom of a bog, some twenty or thirty feet in depth, is 
often so compacted that when dried it resembles lignite. In 
like manner, thick artificial accumulations of loose rock-rubbish, 
as everyone knows, become in time sufficiently consolidated 
to serve as foundations for buildings. When we are assured, 
therefore, that many rocks of sedimentary origin, now visible 
