62 
THE GKANl) CASfOH DKSTRICT. 
Erosion viewed in one way is the supplement of the process by which 
strata are accumulated. The materials which constitute the sti'atified rocks 
were derived from the degradation of the land. This pro])osition is funda- 
mental in geology — naj^, it is the broadest and most comprehensive propo- 
sition with which that science deals. It is to geology what the law of gravi- 
tation is to astronomy. We can conceive no other origin for the mateilals 
of the strata, and no other is needed, for this one is sufficient and its verity 
a thousand times proven. Erosion and “sedimentation” are the two half 
phases of one cycle of causation — the debit and credit sides of one system 
of transactions. The quantity of material which the agents of erosion deal 
with is in the long run exactly the same as the quantity dealt with by the 
agencies of deposition ; or, rather, the materials thus spoken of are one and 
the same. If, then, we would know how great have been the quantities of 
material removed in any given geological age from the land by erosion, we 
have only to estimate the mass of the strata deposited in that age. Con- 
strained by this reasoning the mind has no escape from the conclusion that 
the effects of erosion have indeed been vast. ' If then these operations have 
achieved such results, our wonder is transferred to the immensity of the 
periods of time required to accomplish them; for the processes are so slow 
that the span of a life-time seems too small to render those results directly 
visible. As we stand before the terrace cliffs and try to conceive of them 
receding scores of miles by secular waste, we find the endeavor quite use- 
less. There is, however, one error against which we must guard ourselves. 
We must not conceive of erosion as merely sapping the face of a straight 
serried wall a hundred miles long ; the locus of the wall receding parallel 
to its former position at the rate of a foot or a few feet in a thousand years 
the terrace back of its crest line remaining solid and uncut ; the beds thus 
dissolving edgewise until after the lapse of millions of centuries their termi- 
nal cliffs stand a hunch’ed miles or more back of their initial positions. The 
true story is told by the Triassic terrace ending in the Vermilion Cliffs. This 
terrace is literally sawed to pieces with canons. There are dozens of these 
chasms opening at intervals of two or three miles along the front of the 
escarpment and setting far back into its mass. Every one of them ramifies 
again and again until they become an intricate net-work, like the fibers of 
