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MAZATZAL QUARTZITE 
members as muds upon wide delta flats which, when the flood 
subsided, were exposed to the sun and air, allowing tension- 
cracks to form. Then the next flood, or else a seasonal tide, 
swept sands over the flat, filling and preserving the cracks. 
Many ripple-marks, usually associated with the slates and shales, 
were noted within the formation. Most of these measured less 
than six inches from crest to crest; but one observed, in the Nat¬ 
ural Bridge area, measured seven feet from crest to crest and 
nine inches of depth in its trough. They may have been formed 
during the inundation which preserved the tension cracks men¬ 
tioned above; and the gigantic ones may have been formed by a 
great tidal wave rather than by a river flood. 
The general lack of limestone in such a thick series of sedimen¬ 
tary rocks is somewhat of a negative criterion. However, it does 
argue, along with the sun-cracked, ripple-marked slates and shales, 
for shallow-water deposition. Furthermore, if a deposit be of 
delta origin, limestones are not to be expected. 
Likewise, it seems natural that the conditions for life in the 
waters of a Pre-Cambrian delta, while by no means prohibitive, 
would not be distinctly favorable. Nevertheless, it is hoped that 
a more diligent search for traces of organic life within the Mazat- 
zal quartzites themselves may throw more light upon this phase of 
the problem. 
