8 
THE GRAND CASTON DISTRICT. 
ever since. It has been the main track along which the waste of the prov- 
ince has been carried to the Pacific. At its beginning its bed lay in Eocene 
strata, and as the land rose it cut down its channel by corrasion, severing 
in succession all the beds of the Mesozoic and Carboniferous systems 
That portion of it which constitutes the Grrand and Marble Canons has cut 
through 10,000 to 16,000 feet of beds, reaching a maximum amount in the 
Kaibab. The present Grand Canon represents only the corrasion through 
the Carboniferous and into the Archaean. The older corrasion of superior 
beds becomes manifest only when we restore in imagination the Mesozoic 
strata which have been denuded from the vicinity of the chasm. The pres- 
ent Grand Canon therefore is the work of late Tertiary and Quaternaiy 
time. Although we cannot fix with precision the exact epoch at which the 
river first penetrated the Carboniferous beds, we may in roughly approxi- 
mate language place that epoch near the beginning of the Pliocene or close 
of the Miocene. These terms, however, are used with considerable latitude. 
Some critical events in the subsequent development of the chasm are pointed 
out and the evidences cited. 
Chapters XIII and XIV deal with the mechanical laws, forces, and 
methods of action by which the canon has been corraded and eroded, and 
explain how those abnormal architectural forms so abundantly displayed 
in the chasm and in the region roundabout have been generated. 
