duttoh.] THE GREAT DENUDATION. 09 



though very great, are not so excessive as to impose such a heavy burden 

 upon the credulity as the first announcement of the figures portended. 



In drawing inferences from the stratification the geologist is obvi- 

 ously bound to presume that the strata cut oft' in the terraces extended 

 originally without a break until they reached some locality where the 

 conditions of deposition failed. There are two, and only two, cases to 

 be considered. The first case is that in which the extension is towards 

 the shore line of the sea or lake in which the strata were deposited. At 

 the shore line the strata, of course, end. In the present case no shore 

 line could have existed southward, between the terraces and the Aubrey 

 Cliffs, beyond the San Francisco Mountains. This is quite certain. We 

 know the countrvsowell that if there had been such a shore line in this 

 interval its traces would have been discovered. AYe are quite sure that 

 no such traces exist. The second casearises when sediments gradually 

 thin out seawards and either vanish entirely or become so thin that 

 their bulk is only nominal. We have already noted that the strata in 

 the terraces (p. 79) grow thinner from west to east, and we know that 

 the shore line of the marine basin, in which they were deposited, lay to 

 the west and northwest. But here we are considering their extensions 

 towards the south, and we already know that more than one hundred 

 miles in that direction was another part of the shore line surrounding 

 the basin trendingnorthwest and southeast. Supposing strata to atten- 

 uate as they recede from, and to thicken as they approach, their shore lines, 

 the case we are considering would perhaps be about as follows. South- 

 ward as far as the Grand Canon, i. c, halfway or thereabout between 

 the terraces and the southern shore, there might be some slight reason 

 for inferring a very little attenuation, but beyond the Grand Canon we 

 might with equal reason infer a thickening. But this reasoning is ob- 

 viously precarious, since the attenuation of strata as they approach or 

 recede from shore lines does not follow any rigorous law — does not con- 

 form to any definite proportion. The best and apparently the only use 

 we can make of it is rather of a negative character, leading us to infer 

 merely that the stratification does not offer any reason for presuming 

 that their original southward extensions were notably thinner than the 

 portions preserved in the terraces. But there is another class of facts 

 which is somewdiat more to the purpose. 



Of the denuded formations, some outliers are preserved at a consid- 

 erable distance from the terraces. In the case of the Permian there is 

 no doubt. The great Carboniferous platform of the Grand Canon dist net 

 is spotted in many places with Permian remnants, though rarely is the 

 whole series preserved. One important remnant shows very nearly the 

 whole series — at Mount Logan, in the Uinkaret Plateau, near the Grand 

 Canon. A conspicuous knoll, called the Red Butte, south of the Kaibab 

 and about 30 miles from the San Francisco Mountains, also preserves a 

 large part of the series, and innumerable patches of lower Permian beds 

 are found on both sides of the great chasm. They show no attenuation 



