CHANGES OP LEVEL. 717 



gieal Eeport and illustrates them with this and other figures, at- 

 tributes the plications to lateral pressure while jfche layer was in a 

 softer state than those contiguous. Its porous condition may have 

 caused it to soften with water more easily than those above or below. 



3. The gravity of wet clayey or sandy beds when in an inclined position 

 (p. 650). — The laminated clay -layers are often plicated by this means, 

 as by the preceding ; and some plications in metamorphic rocks are 

 of this origin. 



4. The undermining of strata by volcanic ejections (p. 699). 

 Other causes are either local or general: — 



5. The formation, more or less sudden, of vapors within or beneath some 

 portion of the earth's crust. — Disturbances in volcanic regions are in 

 part due to this cause. When in the uplifting and fracturing of 

 the rocks by this means they become so wedged as not to fall back 

 to their former position on the condensation or escape of the gases, 

 a permanent elevation is occasioned. 



This cause may produce effects over limited areas. It is often re- 

 garded as a prominent means of lifting mountains and continents. 

 But mountain-chains are heavy, and continents very heavy ; and such 

 vapors, if formed, could at the most only shake the rocky crust. 

 Mountain-chains and continents could not be sustained long on a 

 bed of vapors. For permanent elevation, there must be some 

 mode of holding them up after the uplift. Moreover, there is no 

 reason to believe in the existence of the cavities beneath requisite 

 for the spread of the vapors. 



6. Weight of accumulating formations over extended areas of the earth 7 s 

 surface, producing a subsidence of the crust. — Whether this is an 

 actual cause or not in geological dynamics, is questionable. The 

 great subsidences of the Appalachian region have been attributed 

 to it. But this same Appalachian region underwent oscillations 

 upward as well as downward ; and the former require a very differ- 

 ent cause. It was finally, after long ages of preparation, the scene 

 of disturbances and foldings for a length of 1000 miles and a 

 breadth of some hundreds ; and these effects of continental extent 

 are not results of simple gravitation. It is probable that all the 

 oscillations of level, and the ultimate plication of the crust over 

 the great region, have one common cause ; yet it is not impossible 

 that gravitation may have been one cause of the subsidences. 



7. Movements in the interior fluids of the globe. — If the interior of the 

 earth has been through the geological ages in a state of free liquidity, 

 there must have been tides in the molten sea which, in times of ex- 

 cessive height, might have caused vibrations of the crust ; while if 

 the condition was that of dense viscidity, such a result could not 



