JOO 
THE GEAND CASTOE DISTKICT. 
begun. The denudation of the Mesozoic system was an incomparably 
greater work, and yet that denudation could not have begun until the last 
strata (the Lower Eocene) were deposited. If these inferences are well 
founded, we may assign the greater part of Eocene and the whole of Mio- 
cene time for the principal denudation of the Mesozoic, and the Pliocene 
and Quaternary for the excavation of the entire canon. The proportion 
thus suggested between the portions of the work done and the divisions of 
time required to accomplish them seems very fair and reasonable. But the 
strongest evidence of all it would be almost impossible to recite here in 
detail. In general terms, it may be characterized as that internal evidence 
which appears when a vast array of facts, at first disjointed and without 
obvious relation, are subsequently grouped aright into a coherent S5"stem. 
Each constituent fact is then seen to admit of one intelligible interpretation 
and no other; and each subsidiary proposition has an overwhelming justifi- 
cation and an evidence of verity far stronger than any which could be 
summoned, if we endeavored to prove it independently. 
