254 
THE GRAND CANON DISTRICT. 
acter, it yields readily to weatliering. Much of the silica has been aggre- 
gated into the form of cherty nodules, which are very abundant, forming, 
indeed, a considerable percentage of the entire mass of these beds. The 
nodules are to a considerable extent arranged in horizontal bands, occurring 
at frequent intervals, with separating layers of sandy limestone, in which 
the nodules, though still numerous, are less frequent. In the process of 
weathering, the nodules are less easily dissolved than the inclosing matrix, 
and as the rock decays they are left projecting from its mass, giving the 
faces at a little distance the appearance of a bedded conglomerate Ulti- 
mately they are detached and fall down upon the talus below. Millions of 
these nodules are found in the great talus across the edges of the lower 
Aubrey. 
The thickness of the upper Aubrey limestones (of which there are 
several members of somewhat varied constitution) is altogether about 700 
feet. Beneath them comes the hardest and most obdurate mass of the entire 
Carboniferous series — the cross-bedded sandstone. Its thickness is about 
350 to 375 feet. It forms a vertical ledge which is seldom broken into a 
slope. It is seen as a most conspicuous band in the summit wall in every 
amphitheater and promontory. Beneath it is a vass mass of i-ather thinl}^ 
bedded sandstones, constituting the lower Aubrey group. Their total thick- 
ness exceeds a thousand feet. These sandstones are the most yielding of 
aiay portion of the Carboniferous series, since the cement which holds the 
mass together is to a notable extent gypsiferous. It seldom forms a great 
cliflF, but is in reality a succession of ledges often imperfect and bev- 
eled ofiF, but sometimes well marked and precipitous. The individual 
ledges are of very small altitude, corresponding to the thickness of the 
several members. They are to a considerable degree protected by the 
nodules of chert and the slabs of hard sandstone shot down from the cal- 
careous members and cross-bedded sandstone above. In the vast expanse 
of wall surface the individuality of the separate ledges disappears, and the 
general effect conveyed by the lower Aubrey is that of a long, steep, and 
regular slope. When seen in profile the true slope is readily appreciated ; 
but when viewed directly in front, or with moderate obliquity, it looks too 
steep for human foothold, though it is by no means so. 
