14 STUDIES, SCIENTIFIC AND SOCIAL chap 



then," he remarks, " occurs the startling difficulty, why 

 has the sea worn out these great, though circumscribed, 

 depressions on a wide platform, and left mere gorges, 

 through which the whole vast amount of triturated matter 

 must have been carried away ? " Finally, he suggests, 

 that marine currents often form banks of most irregular 

 form, and so steep that a small amount of subsequent 

 erosion during elevation might form them into cliffs. We 

 must consider, however, that this plateau has certainly 

 been elevated since the latter part of the secondary period, 

 leaving ample time for any amount of denudation ; and 

 Mr. Beete-Jukes, in his Sketch of the Physical Struchtre of 

 Australia, informs us that similar valleys abound through- 

 out the great sandstone formation, both at high and 

 low levels ; and they have so exactly the character, in the 

 distribution of their diverging branches, of ordinary streams 

 carrying off the drainage of a slightly inclined surface, that 

 no exceptional origin for them seems needful. This will 

 be more clear when we have discussed the modern theory 

 of valley-formation and the special characteristics of 

 the rocks in which these remarkable valleys have been 

 excavated. 



Hoio Valleys are Formed. 



One of the most common ideas, when a person sees a 

 deep gorge or ravine bounded by lofty precipices, is, that 

 the rocks have been torn asunder by some earthquake or 

 other subterranean movement. A " convulsion of nature " 

 is almost always referred to in popular descriptions of such 

 scenes. Till recent years even geologists considered that 

 many valleys were so formed. The article on the " Geology 

 of the Alps," by M. Desor, in Ball's Alpine Guide, pub- 

 lished in 1870, gives "valleys of disruption" as one of the 

 forms of Alpine valleys, and cites the defile of the Via 

 Mala on the Hinter Rhein, and the valley of the Rhor.e 

 between Bex and Martigny, as examples. He defines 

 them as "evidently produced by rents that have torn 

 asunder ranges once continuous." Professor Whitne}^, 

 also, in his Yosemite Guide- Book, speaks of rents or fis- 

 sures as one of the recognised modes of valley-formation. 



