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 Sedgwick, 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 of this idea ingeniously proposed by Sir John Herschel, that this 

 molecular force was of the nature of an incipient crystallization, and has been 



