THE CENTURY'S PROGRESS IN GEOLOGY 



It became clear that the supposedly stable land sur- 

 faces are in reality much more variable than the surface 

 of the " shifting sea" ; that continental masses, seeming- 

 ly so fixed, are really rising and falling in billows thou- 

 sands of feet in height, ages instead of moments being 

 consumed in the sweep between crest and hollow. 



These slow oscillations of land surfaces being under- 

 stood, many geological enigmas were made clear such 

 as the alternation of marine and fresh-water formations 

 in a vertical series, which Cuvier and Brongniart had 

 observed near Paris; or the sandwiching of layers of 

 coal, of subaerial formation, between layers of subaque- 

 ous clay or sandstone, which may be observed every- 

 where in the coal measures. In particular, the extreme 

 thickness of the sedimentary strata as a whole, many' 

 times exceeding the depth of the deepest known sea, 

 was for the first time explicable when it was under- 

 stood that such strata had formed in slowty sinking 

 ocean-beds. 



AH doubt as to the mode of origin of stratified rocks 

 being thus removed, the way was opened for a more 

 favorable consideration of that other Huttonian doc- 

 trine of the extremely slow denudation of land surfaces. 

 The enormous amount of land erosion will be patent to 

 any one who uses his eyes intelligently in a mountain 

 district. It will be evident in any region where the 

 strata are tilted as, for example, the Alleghanies 

 that great folds of strata which must once have risen 

 miles in height have in many cases been worn entirely 

 away, so that now a valley marks the location of the 

 former eminence. Where the strata are level, as in the 

 case of the mountains of Sicily, the Scotch Highlands, 

 and the familiar Catskills, the evidence of denudation is, 

 i 129 



