240 © BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
a definite period, no matter how short, passes through a cycle and can 
be described in terms of age and development. 
Referring again to the diagram, it will be seen that the three periods 
of erosion that are discussed in this paper —the pre-fault, inter-fault, and 
post-fault or canyon cycles—are not rigidly limited by hard and fast 
boundaries. They shade off into one another, and no one can say exactly 
where one ends and the other begins. Nor can we say that in any two 
of them erosion has proceeded to exactly the same extent, for the 
two cycles that have come to an end were interrupted long before they 
had been carried anywhere near ideal completion, and the present cycle 
is still very little advanced. In certain places, to be sure, the inter- 
fault cycle had proceeded so far that the country was in the final stage 
of development, old age, but elsewhere either because the strata were 
harder or were less exposed to attack, the same length of time had only 
been sufficient to reduce it to the middle of maturity. Yet both these 
types fit perfectly into the cycle scheme of treatment. So, too, if to-day 
in its extremely youthful condition the plateau region should sink helow 
sea-level, the canyon cycle would still have all the attributes of a true 
cycle in the second of the senses defined above. 
One further feature illustrated by~the diagram deserves attention. 
A period of faulting, like a cycle of erosion, seems to involve a definite 
series of systematic changes. From a feeble, perhaps imperceptible be- 
ginning, it, theoretically at least, passes through a period of increasing 
strength until the rate of dislocation reaches a maximum. It then 
becomes gradually weaker and gradually dies out. If it would not lead 
to confusion, these stages, as well as those of the cycle, might fitly be 
termed youth, maturity, and old age; but it must be clearly borne in 
mind that when faulting has attained old age or death, the cycle of ero- 
sion that it introduced may still be in early youth or at most can only 
be in maturity. 
In considering any period of uplift it is necessary to take into account 
not only the processes and changes which are represented by the vertical 
component of the diagram (Fig. 9), but also of those features which are 
represented by horizontal space. Accordingly a complete treatment of 
a displacement such as that of the Hurricane fault involves three main 
heads. ‘(1) The first is the condition of the country before the faulting. 
Were the strata horizontal or plicated? Was the surface above or below 
sea-level? If it was above sea-level, had it been subjected to little or 
much erosion? and are any traces of the pre-faulting topography pre- 
served from which we can reconstruct the whole? (2) The second con- 
