NOETHWAED DIP OF TEERAOE FOEMATIONS. 
47 
the lower and more southerly terraces greater — than they would be if the 
whole series were horizontal. In the entire series of beds which are exposed, 
the aggregate thickness from the top of the Carboniferous to the summit of 
the local Eocene is not far from 10,000 feet, but the summit of the Eocene 
at present lies only about 5,000 to (i,000 feet above the Carboniferous plat- 
form of the Grand Canon district. Thus, if the strata were horizontal, we 
should in ascending the terraces go up 10,000 feet, but the dip to the north- 
ward gradually carries down the horizons so that in crossing the edges of 
10,000 feet of strata we only gain 5,000 to 6,000 feet in altitude. We find 
this same northward dip prevailing in the Carboniferous to the southward, 
and it is a feature of great moment in the studies which are to follow. 
Looking a little more in detail we find a very striking group of subor- 
dinate facts in connection with these dips. At the base of each terminal 
cliff the dip suddenly increases for a short distance and still further south- 
ward diminishes again. The strata in the median parts of any given ter- 
race are very nearly horizontal and have inclinations scarcely exceeding 
one degree. But as we cross such a terrace northwards and approach the 
front of the wall terminating the next higher terrace the inclination becomes 
three or four degrees, and the beds on which we stand are seen to descend 
beneath the talus or alluvial slope in front of the wall. The mind here in- 
stinctively suggests that this may be due to a general settling down of the 
earth beneath the abrupt increase in the gross weight of the great bodies of 
superposed strata. Or, inversely, that the removal of a corresponding mass 
in front of the trenchant cliff-lines has permitted in some measure the asser- 
tion of the laws of plastic equilibrium. The northward dip of the strata 
and their local increments at the bases of the cliffs is represented in the 
sections (Plate III). 
ATTENUATION OF THE STRATA TOWARDS THE EAST. 
Another point to be noted is that the strata slowly diminish in thick- 
ness from west to east. The attenuation, however, is ordinarily very slow 
and gradual, and the observer would have to travel many miles along the 
