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_Studies in Structural Geology._ By =Bailey Willis=, U. S. Geol.
Surv. (Transactions of the American Institute of Mining
Engineers, June, 1892).
The paper aims "to present some of the results of observation of the
geologists of the Appalachian division during the past three years
on the subject of structural geology in the Appalachian province."
The structural features are all of one type but of different phases,
comprised in four great districts. 1) the district of close folding,
2) a district whose chief structural characteristic is cleavage, 3) a
district of open folding, 4) a district of faulting and folding. The
answer to the questions, Why did the strata bend in the district of
open folding, and why did they break in the district of faulting, is
that the thrust affected them according to their rigidity under their
respective conditions of superincumbent load. "We know that load up
to a certain point restrains fracture in material under thrust." In
the district of open folding the Devonian limestone is the most rigid
of the strata and "the one which would most effectively transmit the
compressing thrust and would control the resulting structure." In the
district of open folding this limestone was prevented from breaking
and faulting by a load of superincumbent strata exerting a pressure of
10,000 to 23,000 pounds per square inch, while in the faulted district
a load of 5,000 to 10,000 pounds per square inch permitted the strata
to break and fault.
The answer to the question, Why did the compression affect this
zone, is given. "It becomes apparent on study of sections that where
compression raised a great arch there previously existed a bend from
a nearly horizontal to a descending position in the principal stratum
transmitting the thrust. Greater anticlines and synclines originated in
upward and downward convexity of initial dips, due to unequal deposits
of sediments which depress underlying strata in proportion to their
weight. Such folds may be called original." The Pottsville, Mahanoy,
Shamokin and Wyoming coal basins of Pennsylvania belong to this class.
Experiments have recently been carried on in the office of the United
States Geological Survey reproducing the different forms of folding.
The experiments differed from other experiments in that 1) the
materials used to simulate the stratified rocks varied in consistency
from brittle to plastic, according to the depth at which deformation is
supposed to take place; 2) the compression was exerted under a movable
load representing the weight of superincumbent strata; 3) the strata
rested on a yielding base to simulate the condition of support of any
arc of the earth's crust. The following are the conclusions from the
experiments:
1. "When a thrust tangentially affects a stratified mass, it is
transmitted in the direction of the strata, and by each stratum
according to its inflexibility. At any bend the force is resolved into
components, one radial, the other tangential to the dip beyond the
bend; the radial component, if directed downward, tends to depress the
stratum and displace its support.
2. "A thrust so resolved can only raise an anticline or arch which is
strong enough to sustain the load lifted by its development; such an
arch may be called competent; and since strength is a function of the
proportions of a structure, it follows that, for a given stratum, the
size of a competent anticline will vary inversely as the load; or for a
given load the size will vary as the thickness of the effective stratum.
3. "The superincumbent load borne by a competent anticline is
transferred to the supports of the arch at the points of inflection of
the limbs.
4. "When a competent arch is raised by thrust from one side, the load
transferred may so depress the resulting syncline further from the
force that an initial dip will be produced in otherwise undisturbed
strata; this dip will rise to a bend from which a new anticline may be
developed. This anticline is a result of the first, and may be called
'subsequent' in distinction to original folds. Since subsequent folds
are simply competent structures, their size will be determined by
conditions of thickness and load, and for like conditions they should
be equal; and they must, in consequence of conditions of development,
be parallel to the original fold and to each other. An example of an
original fold with its subsequent anticlines is the Nittany arch and
the group of parallel anticlines which lie southeast of it, extending
northeast from the Broad Top basin."