Gt'ologij of JS^oi'thrru Me.i'lco ll'dl. 
139 
them only in aiTangement and greater tilting 
of the strata. They represent the bent and down- 
thrown portion of what is technically known as a 
faulted monoclinal fold as shown in the accom- 
pan3ung figure. This separated them fi'om the 
main mountain mass. 
The strata of the Sierra Chiquita or Hog Backs 
stand almost verticall}', dipping 80° north and 
striking 20° south of east. 13}' erosion the Sierra 
Chiquita which once constituted a continuous 
ridge, have been separated b}' canons and given 
their present serrated and isolated individual- 
ity, as shown in the sketch of the mountain 
range. 
Accompanying the great fault which separated 
the Chiquitas from the Sierra Grande, numerous 
fissures, joints and all the fractures were made 
in these smaller mountains at right angles to the 
strike of their stratification and the main fault, 
and into these at a subsequent period has been in- 
filtrated the mineral and accompanying vein mat- 
ter. By this sub-vertical tilting of the strata of 
the Sierra Chiquita. upward of 5.000 feet of the 
limestone formation are visible in the canons, 
which cross them at right angle to the stratifica- 
tion and in the direction of the veins (north and 
south appj)roximately). 
(r) Thr Lniiiltii>i in- Font I HI In. Tliis range of 
small hills which run [)arallel to the Chiquitas, 
is a product of the same folding w'hich produced 
the latter, l)ut is of a later age and softer strata, 
l)eing composed of a thin laminated calcareous 
shale, of the Upper Cretaceous, which in places is 
very much metamorphosed, having the appearance 
of slate, and so called l)y the miners. In other 
places the}' are almost chalky. It is owing to 
this diflFerence in hardness and structure that 
they have yielded more readily to the erosion 
and hence their diminutive size. .\t the contact 
plane of tliis t'oi'ination of the iioniitos and 
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