1881 .] 
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[Crosby. 
ture of the stratified rocks, it is developed at the same time as 
the rocks and by the same forces, and true stratification is almost 
unknown outside of this class of rocks. Similarly, ripple-marks, 
rain-drop impressions and fossils are original structural features 
of, and are absolutely peculiar to, the stratified rocks. So, also, it 
might be shown that the original structures of eruptive rocks are 
not found in veins or beds, nor those of vein rocks, as a rule, in 
dikes or beds. 
The subsequent structures, on the other hand, are the product, 
for the most part, of forces which, like denudation and the tangen- 
tial pressure due to the cooling and consequent contraction of the 
earth’s interior, affect with more or less uniformity all classes of 
rocks. Therefore, the subsequent, unlike the original, structures 
must be, in a general way at least, common to all classes of 
rocks. It is true that we commonly think of folds, faults, etc., 
only in connection with stratified rocks ; yet a moment’s reflec- 
tion will show that veins and dikes cannot be wholly free from 
these structures. Folds are less common with veins and dikes 
than with stratified deposits chiefly because their normal position 
is vertical instead of horizontal ; but faulted veins and dikes are 
among the most common experiences of miners and field geolo- 
gists. The most natural subdivisions of both the original and 
subsequent structures are dynamical. This is especially evident in 
the case of the original structures because the divisions resting on 
a dynamical basis are, as we have seen, coincident with the 
most important lithological divisions, each great class of rocks 
being the product of a distinct class of agencies. In other words, 
we naturally divide the original rock-structures into three classes, 
as follows : (1) Those peculiar to the stratified rocks ; (2) those 
peculiar to eruptive rocks ; and (3) those peculiar to the vein 
rocks. 
The subsequent structures, which are, in general, common to 
all classes of rocks, should be divided at the outset into two 
classes, according as they have been produced by superficial or 
subterranean agencies. The subterranean forces are both mechan- 
ical and chemical. So far as rock-structures are concerned, the 
mechanical subterranean forces, which are, for the most part, sim- 
ply different manifestations of the great strain developed in the 
