Boox VII.] TERRESTRIAL FLEXURES. 9138 
ocean bed. In the long process of contraction the earth has not con- 
tracted uniformly and equably. There have been no doubt vast periods 
during which no appreciable or only excessively gradual movements 
took place ; but there have probably also been intervals when the accu- 
- mulated strain on the crust found relief in more or less rapid collapse. 
Lhe general result of such terrestrial disturbances has been to 
throw the crust of the earth into wave-like undulations. In some 
eases a wide area has been upheaved as a broad low arch with little 
disturbance of the original level stratification of its component rocks. 
More usually the undulations have been impressed as sensible de- 
formations of the crust, varying in magnitude from the gentlest 
appreciable roll up to mountainous crests of complicated plication, 
inversion, and fracture. As a rule the undulations have been linear, 
but their direction has varied from time to time, having been deter- 
mined at right angles, or approximately so, to the trend of the 
lateral pressure that produced them. 
Considered with reference to their mode of production, the 
leading contours of a land-surface may be grouped as follows :— 
1. Those which are due more or less directly to disturbance of the 
erust. 2. Those which have been formed by volcanic action. 
&. Those which are the result of denudation. 
1, Terrestrial Features due more or less directly 
to Disturbance of the Crust.—lIn some regions large areas 
of stratified rocks have been raised up with so little trace of curva- 
ture that they seem to the eye to extend in horizontal sheets as 
wide plains or table-lands. If however these areas can be followed 
sufficiently far, the flat strata are eventually found to curve down 
slowly or rapidly, or to be truncated by dislocations. In an elevated 
region of this kind, the general level of the ground corresponds on 
the whole with the planes of stratification of the rocks. Vast regions 
of Western America, where Cretaceous and later strata extend in 
neatly horizontal sheets for many thousands of square miles at 
heights of 4000 feet or more above the sea, may be taken as 
‘illustrations of this structure. 
As a rule, curvature is more or less distinctly traceable in every 
region of uplifted rocks. Various types of flexure may be noticed, 
of which the following are some of the more important. 
(a.) Monoclinal Flevures (p. 516).—These occur most markedly in 
broad plateau regions and on the flanks of large broad uplifts, as in 
the table-lands of Utah, Wyoming, &c. They are frequently re- 
placed by faults, of which indeed they may be regarded as an 
incipient staze (p. 526). 
(b.) Symmetrical Flexures, where the strata are inclined on the 
two sides of the axis at the same or nearly the same angle, may 
be low gentle undulations or may increase in steepness till they 
become short sharp curves. Admirable illustrations of different 
degrees of inclination may be seen in the range of the Jura and the 
Appalachians, where the influence of this structure of ie rocks on 
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