464 PROF. H. D. ROGERS ON THE LAWS OF STRUCTURE 
temporary flexures, to the overlying crust, and these flexures would be rendered 
permanent (or keyed into the forms they present) by the intrusion of molten matter. 
If, during this oscillation, we conceive the whole heaving tract to have been shoved 
(or floated) bodily forward in the direction of the advancing waves, the union 
of this tangential, with the vertical wave-like movement, will explain the peculiar 
steepening of the front side of each flexure, while a repetition of similar operations 
would occasion the folding under, or inversion, visible in the more compressed 
districts. We think that no purely upward or vertical force, exerted either simul- 
taneously or successively along parallel lines, could produce a series of symmetrical 
flexures, and that a tangential pressure unaccompanied by a vertical force, would 
result only in an imperceptible bulging of the whole region, or an irregular plication 
dependent on local inequalities, in the amount of the resistance. The alternate 
upward and downward movement necessary to enable a tangential force to bend 
the strata into a series of regular parallel subsiding flexures has been, we conceive, 
of the nature of a pulsation, such as would arise from a succession of actual waves 
rolling in a given direction, beneath the earth’s crust. It is difficult to account 
for the phenomena, by any hypothesis of a gradual prolonged pressure exerted 
either vertically or horizontally. The formation of the grand, yet simple flexures 
so frequently met with, cannot be explained by a repetition of feeble tangential 
movements, since these could not successively accord, either in their direction or 
in their amount, nor can it again, by a repetition of merely vertical pressures, for 
it is impossible to suppose that these could, without some undulating action, shift 
their positions through a series of symmetrically disposed parallel lines. We 
find it equally impossible to understand how, if feeble and often repeated, these 
vertical pressures should always return to the same lines to produce the con- 
spicuous flexures we behold. The oscillations of the crust to which the undula- 
tions of the strata are attributed have been, we conceive, of the nature of the 
Earthquakes of the present day. Earthquakes consist, as we think we have de- 
monstrated, of a true pulsation of the flexible crust of the globe, propelled in 
parallel low waves of great length and amplitude with prodigious velocity, from 
lines of fracture, either conspicuous volcanic axes, or half concealed deep-seated 
fissures, in the outer envelope of the planet. 
Theory of Cleavage Structure. 
Concerning the cause of slaty cleavage, I have adopted the explanation origi- 
nally proposed by Professor Sepewick, that it is due to crystalline or polar forces 
acting simultaneously and somewhat uniformly in given directions on large masses 
having a homogeneous composition. And following up the further suggestion in 
extension ofthis idea ingeniously proposed by Sir Jonn Herscuet, that this 
molecular force was of the nature of an incipient crystallization, and has been 
