70 
THE GRAND OANON DISTRICT. 
■with almost rigorous liorizontality, and very nearly at sea-level, throughout 
the entire Mesozoic. This gives us at once a datum horizon of the best 
possible kind to which all subsequent displacements may be referred. 
Its advantage is at once apparent; for it will soon appear very clearly 
that the faults are all of Tertiary age. In estimating the entire displace- 
ment or differential movement involved in a fault, the follov/ing difficulty 
arises. We are often at a loss to ascertain whether one side of the fault 
plane has been lifted or the other depressed, or even whether both sides 
may not have been uplifted or lowered, but one of them more than the 
other; the fault being merely the difference of two movements. But the 
moment we establish the origin of movement at any datum level, this 
problem for any particular fault is solved at once. The faults of the Grand 
Canon district are those which are due to the differences of upward move- 
ment on both sides of the fault planes. This statement is true in every 
instance, and is fully demonstrated. 
The next step is to find the amount and distribution of that part of 
the movement which is represented by the faults. This is done by depress- 
ing the upthrows until the edges of the faulted beds come together and the 
monoclines are smoothed out. If this be done, the sharp ledges and abrupt 
breaks in the topography produced by those displacements disappear. The 
final step is a little more difficult and complex, and it is necessary to de- 
scribe at some length one important feature of the district. 
If we examine the profile of the district from the Markagunt south- 
ward to the Grand Canon, and thence to the Aubrey Cliffs, we shall find 
that the strata all dip very slightly to the north. The amount of this dip 
is irregular in different parts of the profile, being for very short distances, 
and, in a few cases, as much as two and a half or even three degrees; while 
there are long distances where the strata are strictly horizontal. On an 
average it is not far from forty minutes. But since its effect is cumulative 
over the entire distance of 130 to 180 miles, its importance is very great, 
for the Carboniferous beneath the Markagunt lies below sea-level, while at 
the San Francisco Mountains it is nearly 8,000 feet above. The dips are 
strongest near the terraces, and it is very interesting to note the fact that at 
