142 STRUCTURAL AND FIELD GEOLOGY 
their subordinate laminz have become distorted, it is not 
surprising that their individual constituents should similarly 
yield evidence of compression. Thus the rounded stones of a 
conglomerate (Plate X XI. 3) are often flattened against each 
other and drawn out into elliptical or lenticular forms, while 
fossils are frequently distorted in like manner. This process 
of deformation has often proceeded so far as to result in the 
more or less complete alteration of the rocks, the original 
characters being either much obscured or even Jentirely 
obliterated. But the further consideration of such changes 
must be deferred until we come to discuss the phenomena of 
slaty cleavage and metamorphism. 
Origin of Folds.—The various folds described above are obviously the 
result of lateral compression brought about by the sinking of the superficial 
crust of the globe upon the cooling and contracting interior. If we think 
of it, there must be a gradual passage downwards from the cooled crust 
into the still uncooled nucleus. At some depth from the surface, therefore, 
a level will be reached at which the interior has not yet begun to cool 
and contract. Theoretically, we may consider all the matter above that 
level as constituting the crust. The lower section of the crust, reposing 
immediately upon the uncooled nucleus, is cooling and therefore con- 
tracting, and must obviously be in a state of tension to which it will 
seek to yield by rupturing. Not that fissures or rents will actually be 
formed, for the enormous compression exerted by the upper crustal layers 
will necessarily prevent anything of the kind taking place. The stretching 
of the crust by lateral tension must diminish upwards, until a level is 
attained where it will cease altogether. Above that level the crust is 
no longer in a state of lateral tension, but in one of lateral compression— 
it is not stretching but shrivelling. And the lateral compression which 
causes the superficial crustal shell to shrivel increases upwards, and is 
therefore greatest at the surface. There are thus two kinds of contraction 
to which the crust is subjected, namely, circumferential below and radial 
above. When the former is in excess, stretching with tendency to rupture 
is most marked; where the latter prevails, compression is dominant ; 
where the one equals the other there is no strain. It is therefore only 
the crustal shell above this neutral zone or “level-of-no-strain” which is 
liable to become folded. How thick that shell may be we do not know, 
but as the folded rocks in some mountain chains reach a thickness of 
ten miles or more, the upper crustal shell must be of that thickness at 
least. Although folded strata are met with very generally, in Old and 
New Worlds alike, nevertheless we now and again enter regions of great 
extent, over which the strata have retained their original horizontal 
arrangement. It is notable, further, that while gently undulating strata 
often extend throughout vast areas, highly folded and contorted rocks 
tend to occur in zones or belts. It would thus seem that the earth’s 
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