ANTIQUITY OF OUR GLOBE. 149 



making, in our calculations, only a remote and indefinite ap- 

 proximation. During comparatively short periods of violent 

 physical revolution, conglomerates and other coarse and indis- 

 tinctly stratified rocks may, in some instances, have been de- 

 posited with comparative rapidity. Older rocks were prob- 

 ably disintegrated by the combined agency of heat and water, 

 and ground to fragments by volcanic and marine agitation ; 

 and, by violent currents, probably thus generated, they may 

 have been carried to lower levels, and sometimes formed thick 

 deposits in comparatively short periods. But these instances 

 are only exceptions to the general rule, while far the greater 

 proportion of the stratified rocks present unmistakable evi- 

 dence of having been deposited in quiet waters. And these 

 deposits could not, in general, have accumulated much more 

 rapidly than similar ones which are going on at the present 

 time. Now, it is said that the lakes of Scotland shoal, by 

 sedimentary depositions, only at the rate of about six inches 

 in a century.* Making all reasonable allowance for the su- 

 perior activity of early disintegrating and depositing forces, 

 the period which must have been consumed during the depo- 

 sition of materials which have formed rocks of twenty miles 

 in perpendicular thickness, can be estimated only by millions 

 of years, especially when we take into account the long 

 periods of super-marine elevation and repose which sometimes 

 must have intervened between the close of one formation and 

 the commencement of the succeeding one. 



Our conception of the immensity of the periods of these de- 

 posits is augmented when we consider that beds of rocks 

 of great thickness, and sometimes whole mountains, many 

 thousand feet high, are made up almost entirely of sea-shells 

 and other organic matter these mountains having originally 



* Hitchcock's Geology, p. 163. 



