Chapter XIV: Part V: Geotectonic or Structural Geology

From a study of the nature and composition of minerals and rocks, and an investigation of the different agencies by which they are formed and modified, the geologist proceeds to inquire how these materials have been put together so as to build up the visible part of the earth's crust. He soon ascertains that they have not been thrown together wholly at random, but that they show a recognizable order of arrangement. Some of them, especially those of most recent growth, remain in their original condition and position, but, in proportion to their antiquity, they generally present increasing alteration, until it may no longer be possible to tell what was their pristine state. As by far the largest accessible portion of the terrestrial crust consists of stratified rocks, and as these furnish clear evidence of most of the modifications to which they have been subjected in the long course of geological history, it is convenient to take them into consideration first. They possess a number of structures which belong to the original conditions in which they were accumulated. They present in addition other structures which have been superinduced upon them, and which they share with the unstratified or igneous rocks.

1. ORIGINAL STRUCTURES

(a) _Stratified Rocks._--This extensive and important series is above all distinguished by possessing a prevailing stratified arrangement. Their materials have been laid down in laminae, layers and strata, or beds, pointing generally to the intermittent deposition of the sediments of which they consist. As this stratification was, as a rule, originally nearly or quite horizontal, it serves as a base from which to measure any subsequent disturbance which the rocks have undergone. The occurrence of false-bedding, i.e. bands of inclined layers between the normal planes of stratification, does not form any real exception; but indicates the action of shifting currents whereby the sediment was transported and thrown down. Other important records of the original conditions of deposit are supplied by ripple-marks, sun-cracks, rain-prints and concretions.

From the nature of the material further light is cast on the
geographical conditions in which the strata were accumulated. Thus,
conglomerates indicate the proximity of old shore-lines, sandstones
mark deposits in comparatively shallow water, clays and shales point
to the tranquil accumulation of fine silt at a greater depth and
further from land, while fossiliferous limestones bear witness to
clearer water in which organisms flourished at some distance from
deposits of sand and mud. Again, the alternation of different kinds of
sediment suggests a variability in the conditions of deposition, such
as a shifting of the sediment-bearing currents and of the areas of
muddy and clear water. A thick group of conformable strata, that is, a
series of deposits which show no discordance in their stratification,
may usually be regarded as having been laid down on a sea-floor that
was gently sinking. Here and there evidence is obtainable of the
limits or of the progress of the subsidence by what is called
"overlap." Of the absolute length of time represented by any strata or
groups of strata no satisfactory estimates can yet be formed. Certain
general conclusions may indeed be drawn, and comparisons may be made
between different series of rocks. Sandstones full of false-bedding
were probably accumulated more rapidly than finely-laminated shales or
clays. It is not uncommon in certain Carboniferous formations to find
coniferous and other trunks embedded in sandstone. Some of these trees
seem to have been carried along and to have sunk, their heavier or
root end touching the bottom and their upper end slanting upward in
the direction of the current, exactly as in the case of the snags of
the Mississippi. In other cases the trees have been submerged while
still in their positions of growth. The continuous deposit of sand at
last rose above the level of the trunks and buried them. It is clear
then that the rate of deposit must have been sometimes sufficiently
rapid to allow sand to accumulate to a depth of 30 ft. or more before
the decay of the wood. Modern instances are known where, under certain
circumstances, submerged trees may last for some centuries, but even
the most durable must decay in what, after all, is a brief space of
geological time. Since continuous layers of the same kind of deposit
suggest a persistence of geological conditions, while numerous
alternations of different kinds of sedimentary matter point to
vicissitudes or alternations of conditions, it may be supposed that
the time represented by a given thickness of similar strata was less
than that shown by the same thickness of dissimilar strata, because
the changes needed to bring new varieties of sediment into the area of
deposit would usually require the lapse of some time for their
completion. But this conclusion may often be erroneous. It will be
best supported when, from the very nature of the rocks, wide
variations in the character of the water-bottom can be established.
Thus a group of shales followed by a fossiliferous limestone would
almost always mark the lapse of a much longer period than an equal
depth of sandy strata. A thick mass of limestone, made up of organic
remains which lived and died upon the spot, and whose remains are
crowded together generation above generation, must have demanded many
years or centuries for its formation.

But in all speculations of this kind we must bear in mind that the
length of time represented by a given depth of strata is not to be
estimated merely from their thickness or lithological character. The
interval between the deposit of two successive laminae of shale may
have been as long as, or even longer than, that required for the
formation of one of the laminae. In like manner the interval needed
for the transition from one stratum or kind of strata to another may
often have been more than equal to the time required for the formation
of the strata on either side. But the relative chronological
importance of the bars or lines in the geological record can seldom be
satisfactorily discussed merely on lithological grounds. This must
mainly be decided on the evidence of organic remains, as shown in