389 



floated) bodily forward in the direction of the advancing waves, the 

 union of this tangential with the vertical movements may explain the 

 peculiar steepening of the front side of each flexure, while a succession 

 of similar operations will accomplish the folding under or inversion 

 seen in the more compressed districts." They think that no purely 

 vertical force, exerted either simultaneously or successively along paral- 

 lel lines, could produce a series of symmetrical flexures, while a tan- 

 gential pressure, unaccompanied by a vertical force, would result only 

 in an imperceptible bulging of the whole region or in irregular plica- 

 tions, dependent on local inequalities in the amount of the resis- 

 tance. The alternate upward and downward movement necessary to 

 enable a tangential force to bend the strata into a series of regular 

 parallel subsiding flexures was, they conceive, of the nature of a pul- 

 sation such as would arise from a succession of actual waves roll- 

 ing in a given direction beneath the earth's crust. Successive 

 feeble tangential movements could not agree either in direction or 

 amplitude, nor is it easy to imagine how they could shift their posi- 

 tions through a series of parallel axis lines, nor how, when renewed, 

 they could return always to the same lines to build up the conspicuous 

 flexures. These oscillations of the crust, to which the undulated 

 stjata are attributed, have been, they conceive, of the nature of the 

 earthquakes of the present day ; — earthquakes being, as they have 

 demonstrated, a true pulsation of the flexible crust of the globe, 

 propagated 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 the Origin of Cleavagk Structure. 



Concerning the cause of slaty cleavage, the author of the paper 

 has adopted the explanation originally proposed by Professor Sedg- 

 wick, that it is due " to crystalline or polar forces acting simultane- 

 ously and somewhat uniformly, in given directions, on large masses 

 havin<» 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 developed in the particles by 

 their being heated to a point at which they could begin to move among 



VOL. III. 2 K 



