192 Miscellaneous. 



soft Chalk of the North and South Downs form narrow gorges below 

 the broad alluvial flats of the softer "Weald Clay. But these Weald 

 Clay flats are at the same level as the beds of the Chalk gorges. 

 There are no hollows or lakes above the gorges. 



The origin of all alluvial plains, properly so-called, is the stoppage 

 of the lowering of the bed of the valley. The bed of the valley 

 above the stoppage is then cut back perfectly horizontal at the level 

 of the stoppage. The rain flood- water from the inclined sides of the 

 valley is then checked, overflows and deposits on the horizontal part. 

 The sea stops the lowering of the bed of every valley. Therefore, 

 the parts next the sea are composed of horizontal alluvium. Take 

 the alluvial plain of the Nile from Cairo to Syene. We know that 

 it is raised by deposit every year. But this rising is not the result 

 of a lake " behind a barrier." This rising of the lowest or marine 

 alluvial plain is constant, that is, it will go on as long as the relative 

 level of the land and of the sea remain the same, and no terraces 

 will be formed. Parallel terraces are formed by patches of alluvial 

 plain. That is patches formed in valleys cut in soft strata above 

 gorges of hard strata, which make temporary stoppages of the lower- 

 ing of the bed of the valley. But we do not require (as Mr. Kina- 

 han supposes) " power to scoop out rocks behind a barrier " lower 

 than the barrier. No hollow or lake is formed. The alluvial flat 

 above the gorge is never lower than the bed of the gorge, it is at the 

 same level, or, if anything, a shade higher. This principle accounts 

 for the Kames at Carstairs above the gorges of the Clyde at Lanark, 

 and of the Mouse Water at Cartland Crags, and I guess it would explain 

 the enigma of the Eskers of Central Ireland. 



G-EORGE Geeenwood, Coloncl. 

 Brookwood Park:, Alkesford, Gih March, 1869. 



GEOLOGY OF ALASKA TEEEITORY. 



Mr. Henet Waltee Bates, Secretary to the Eoyal Geographical 

 Society, has kindly forwarded me the subjoined extract from a letter 

 of Mr. W. H. Dall, Smithsonian Institute, Washington, U.S. to F. 

 Whymper, Esq., Haslemere, Surrey: — ■" Alaska. — You can tell your 

 scientific friends that I have settled the geological question by fossils 

 which I got this last year near Topanica (Norton Sound) : a fine 

 species of Platanus, which is undoubtedly Miocene Tertiary; there 

 are no older rocks below Nuclukayette (Yukon Eiver). The south 

 flanks of the Alaskan range have Triassic? and Miocene Tertiary 

 beds." — Mr. Call's large collections are now being arranged at the 

 Smithsonian Institute. 



