340 NATURAL SCIENCE. may. 



According to Ramsay, these islands formed part of a " vast continent, 

 to which the British Islands were united, and which, embracing Ice- 

 land, spread far to the north and west into the area of what is now the 

 Atlantic, and on the south was united to Africa, when as yet the 

 Mediterranean had no existence." '^ 



Long before the extreme denudations represented by these frag- 

 ments of a once continuous sheet took place, old rivers intersected 

 this ancient land and scooped out valleys in the Miocene lavas and 

 hills, which were again partly filled by torrents of basalt and obsi- 

 dian. "Thus it happens that in the old volcanic plateaux, valleys 

 a thousand feet deep have been excavated, and the whole region has 

 by denudation been changed into a line of fragmentary islands the high 

 sea-cliffs of which attest the greatness of the waste they have in time 

 undergone, "^^ Sir A. Geikie'5 says, speaking of Eigg, " Lastly, from 

 the geology of this interesting island we learn, what can be nowhere in 

 Britain more eloquently impressed upon us, that, geologically recent 

 as that portion of the Tertiary period may be during which the 

 volcanic rocks of Eigg were produced, it is yet separated from our 

 own day by an interval sufficient for the removal of mountains, the 

 obliteration of valleys, and the excavation of new valleys and glens 

 where the hills then stood." Though we may not claim all this denuda- 

 tion for Quaternary time, since much of it may have taken place during 

 the Pliocene with resultant Pliocene rocks now sunk beneath the sea, 

 these quotations from eminent geologists ma}^ well serve to give us an 

 inkling of the powers we are dealing wuth. According to Dana, the 

 length of the Quaternary period up to now is one-third of the Tertiary.'^ 

 This is but an approximate guess, but nevertheless a valuable one. 



Turning our attention to North America, we may say that the 

 Mississippi is at least as old as the Quaternary, and probably very 

 much older. The elevation of the Rocky Mountain regions compelled 

 the drainage of the continent to take a south-western course, while 

 the older land of the Laurentian Highlands and the Appalachians 

 blocked it from direct connection with the Atlantic except in the 

 northern portions. It has been shown that since the elevation of the 

 Uinta Mountains, which began in Cretaceous times, 3^ cubic miles of 

 rock have been removed from every square mile of their surface. ^7 

 A large area of the central Mississippi Valley is occupied with 

 Cretaceous rocks, while the southern part, bordering the Gulf of 



IS •■ Physical Geology and Geography of Great Britain," 5th ed., p. 263. 



14 " Physical Geology and Geography of Great Britain," 5th ed., p. 356. 



15 "On Tertiary Volcanic Rocks, ' Q.J.G.S., vol. xxvii., p. 310. 



i" " Manual of Geology," 2nd ed., p. 5S6. 



17 " Origin of Mountain Ranges," p. 243. In a most interesting paper by Dr. 

 Andrew Lawson, entitled, "The Post-Pliocene Diastrophism of the Coast of Southern 

 California" (Bulletin of Dept. of Geol. University of California), it is shown that 

 Pliocene sediments of over a mile thick, called the Merced series, were laid down on 

 the Californian coast. This is an extraordinary example of the accumulation that 

 has taken place in only the closing phases of the Tertiary period. 



