INACCESSIBLE VALLEYS 19 



geneous mass of hard blue rock. Yet the date of the 

 eruption which produced this lava stream is known to be 

 1603.^ But the most wonderful example of the power of 

 water to denude and erode the hardest rocks is afforded 

 by the great canon of the Colorado river. This has been 

 cut for about 400 miles to a depth of from 4,000 to 7,000 

 feet, mainly through masses of hard palaeozoic rocks down 

 to the archsean, and the whole of this vast operation has 

 been performed in the latter half of the tertiary period. 

 The formation of the river began, it is true, in very early 

 tertiary times, but at that epoch the present surface was 

 buried about 9,000 feet deep in secondary rocks, which 

 have all been since denuded away, so that Captain Button 

 estimates that the river has cut its channel on the whole 

 through from 10,000 to 16,000 feet of mesozoic, carboni- 

 ferous, and other ancient rocks, all during the tertiary 

 period.2 



Formation of the Inaccessible Valleys. 



Keeping in mind these remarkable instances of denuda- 

 tion, let us turn to consider the probable origin of the re- 

 markable valleys which have seemed to eminent geologists 

 so peculiar as to need some special mode of origin ; and 

 we will take first the great rock-walled valleys of New 

 South Wales, as being the most simple in their main 

 features. 



These are all excavated in sandstones and shales of the 

 carboniferous system, though perhaps of mesozoic age. 

 The strata are nearly horizontal, and, what is especially 

 important, they are of very unequal degrees of hardness. 

 The upper beds are usually conglomerates, and are so 

 comparatively indestructible that isolated summits often 

 imitate ruined castles. In places these beds are so hard 

 that boring-tools will not penetrate them, while in other 

 parts the rock is so incoherent that large blocks will break 

 in pieces by falling over an embankment.^ We have here 



^ Principles of Geology, vol. i. , p. 353. 



"^ The Tertiary History of the Grand Canon District, U.S. Geological 

 Survey, 1882. 



' Remarks on the Sedimentary Formations of New South JVales. By 

 the Rev. W. B. Clarke, F.R.S., F.G.S. Fourth ed, 1878, p. 72. 



C 2 



