
STRATIFICATION 107 
are invariably much compacted, and sometimes so altered 
as to become truly crystalline and schistose. On the other 
hand, strata which have not been disturbed, but still retain 
their original horizontal position, are usually much less 
hardened, and rarely show any trace of metamorphism. The 
older Paleozoic rocks of this country, for example, are 
usually highly flexed and folded, and not only much com- 
pressed and hardened, but frequently rendered crystalline 
and schistose. In Central Russia, on the other hand, strata 
of the same age have retained their original horizontal 
position, and are so unaltered in general aspect as to resemble 
the sedimentary accumulations of comparatively recent times. 
The contrast between the undisturbed and more or less 
incoherent Eocene deposits of the London Basin and their 
much flexed and folded representatives in the Alps of 
Switzerland, is not less striking. Many similar contrasts 
might be cited, but it is enough to ernphasise the fact 
that great crustal deformation is invariably accompanied 
by the induration of the rocks affected, no matter what 
ies age may be. We may-conclude, therefore, that 
the pressure induced by crustal movements has been 
one of the most effectual and widely acting causes of rock- 
consolidation. 
Lamination and Stratification.—The most abundant and 
widely distributed rocks of derivative origin are undoubtedly 
the sedimentary types, conglomerate, sandstone, and _ shale. 
They have been spread out by the sorting action of water, 
and consequently occur in sheet-like form. Coarse gravel 
(conglomerate) has obviously been deposited upon beaches, 
or in shallow water at the mouths of rapid torrents, streams, 
and rivers. It may therefore be fluviatile, lacustrine, estuarine, 
or marine. In like manner sand (sazdstone) and argillaceous 
sediments (clay, shale, etc.) are of both marine and freshwater 
origin. Sometimes a sedimentary rock has been deposited 
more or less rapidly ; in other cases the process of sedimenta- 
tion has been gradual and protracted. In the latter case, 
this is shown by the structure of the sheet-like deposit, which 
is usually composed of successive layers or very thin lamine. 
In a deposit more rapidly accumulated, this structure is either 
inconspicuous or wanting. 
