xllv IXTRODUCTION. 



1 



I 



miles), and a great, thougli yet unmcasiircd, extension north ^ 

 and soiitli, is occupied by several thousand feet of Pakeozoic 

 and secondary strata, we must conclude that tliesc sedimonta^ 

 have not been derived from the erosion of immcrged surface^" 

 east of the Mississippi, but were here formed by the incessant 

 action of the Pacific waves on shores that, perhaps for 

 hundreds of miles, succumbed to theii' power, ^nd by broad 

 and rapid rivers, which flowed from the mountains, aud 

 through the fertile valleys of a primeval Atlantis.'' 



These many thousand feet of sedimentary strata were 

 converted into dry land by the gradual upheaval of tlie 

 Plutonic rocks upon which they were deposited. Generally* 

 they were raised with but little disturbance of their original 

 positions ; still, districts, or rather lines, of more powerful , 

 upheavals can be traced across the country by the increased 

 height of the table -lands, while here and there more recent ^ 

 volcanic forces have thrust huge masses of igneous rock np 

 through the sedimentary crust, forming mountains more or 

 less isolated, and of great beauty, which contrast strangely i 

 with the eroded mesa-lands amongst which they rise. Su^^ * 

 are the San Francisco Mountains, Mount Taylor, and Bill s 

 Williams Mountains — all now extinct volcanoes. 



The thousand springs of Green and Grand rivers Avhich start , 

 from an elevation of from 10,000 to 12,000 feet, form caiions | 

 in the mountain districts only when some unusual obstacle 

 bars theii- course. In the mountains which give them birtn? 



frequent rains have washed out sloping valleys, and the pru^' 



lit 



tive rocks have generally succeeded in resisting, to a gi'^ 

 extent, their erosive action. 



Further from their sources, beyond the influence of ^^^' 

 mountain rains, these two rivers and their tributaries, in thetf 

 passage over the table-lands of the great central plateau, ha^ ^) 



