Theories of the Formation of Mountains, 413 



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. And, further, the formation of tbe grand yet simple 

 flexures so frequently met with cannot be explained by a repeti- 

 tion of feeble, tangential movements, since these could not succes- 

 sively accord either in their direction or in their amount; nor 

 can it by a repetition of merely vertical pressures, for it is impos- 

 sible to suppose that these could without some undulating action, 

 shift their positions through a seriesof symmetrically disposed par- 

 allel 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 conspicuous flexures we 

 behold. The oscillations of the crust to which the undulations 

 of the strata are attributed have been, we conceive, of the nature 

 of the earth :^uakes of the present day. Earthquakes consist, as 

 we think we have demonstrated, 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 " (^H. D. Rogers^ in 

 the Geology of Pennsylvania. Vol. 2, Pt. 2, p. 911.) 



2. Sir Charles Lyell's Theory. 



Sir Charles Lyell in commenting upon the theory of Prof. 

 Rogers says: — 



" That there were great lakes, or seas of lava, retained by volcanic 

 heat for ages, in a liquid state beneath the Alleghanies, is highly 

 probable, for the simultaneous eruptions of distant vents in the 

 Andes leave no doubt of the wide subterranean areas permanently 

 occupied by sheets of fluid lava in our own times. It is also con- 

 sistent with what we know of the laws governing volcanic action 

 to assume that the force operated in a linear direction, for we see 

 trains of volcanic vents breaking out for hundreds of miles along 

 a straight line, and we behold long parallel fissures, often filled 

 with trap or consolidated lava, holding a straight course for great 

 distances through rocks of all ages. The causes of this peculiar 

 mode of development are as yet obscure and unexplained ; but 

 the existence of long narrow ranges of mountains, and of great 

 faults and vertical shifts in the strata prolonged for great dis* 



