96 J. LeConte—Structure and Origin of Mountains. 
crushing the strata together and thus producing foldings and 
thickening and consequent elevation. In what I shall say of 
mountain-structure I shall be compelled for the sake of clear- 
ness to assume this. I hope to justify this assumption in what 
I shall subsequently say on mountain-origin. 
’ TL. Mountain-Structure. 
Mountain ranges may be conveniently divided into two gen- 
eral classes which, however, graduate completely into each 
other, viz: those which are com posed of a single anticlinal 
fold, and those which are composed of a number of folds 
alternately anticlinal and synclinal, either open, as in the Jura, 
or closely appressed, as in the Appalachian, the Coast Range, 
the Alps, and many others. The one kind is formed where the 
earth crust is more rigid, the other where it yields more readily 
to the horizontal pressure. Both kinds are greatly modified, 
sometimes by eae aE sometimes by faulting, Bea 
y voleanic outbursts, and > ways by subsequent ero 
- Mountains of a single fold.—The simplest concoienble 
mountain range consists x % a single anticlinal fold of a series 
of strata. In such cases the deeper strata of the series are 
thickened and swelled upward by the horizontal pressure, 
while the upper strata are raised into a vault with little or no 
thickening, or may ev e thinned and broken by tension. 
Nearly always the yang’ is greater on one side than on the 
other; so that the vault is unsymmetrical. In such cases a 
great fissure and slip is apt to occur on the steeper mde The 
following figure (fig. 1) is an ideal section of such a mountain 
efore erosion had modified 
its form, or rather (since up- 
swelling and erosion goes "t 
together pari passu) as 1 
would be pean Now 
is evident that in the pir 
would almost certainly be 
formed; and if beneath the 
vault there — exist & 
mass of fused or semi-fused 
— se inaninan males 
er), formed either by the 
invasion of the Poo whe with their included waters, 
by the interior heat of the earth during the preparatory process 
of sedimentation, or by the heat evolved by crushing in the act 
of formation itself of the pe ge dislocations would be apt 
to occur: and further, both the fissures and the faults would 
be most apt to occur just hae the bending of the strata is 
ees on ree: 
an phe ea HAsQUSSOREREED 1009/1 
: TORR 
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