92 GRAND CANON DISTRICT. 



sandstone of very coarse texture, often becoming a conglomerate. Ita 

 thickness is usually from 40 to 75 feet. In a few places it is wanting 

 from its proper horizon, and in some others its thickness becomes more 

 than 100 feet. But on the whole it is a remarkably persistent bed, and 

 its persistence is all the more striking when we consider the coarseness 

 of its texture; for no beds are so variable as the coarse ones. This 

 member has been named by Powell the Shin-a-rump Conglomerate. The 

 name Shinarump he also applies provisionally to a large group of beds 

 in which the conglomerate is included.* For several years it was 

 thought very probable that these beds were a part of the Triassic 

 system, though no positive proof could be cited to sustain that pre- 

 sumption. In the summer of 1879 Mr. 0. D. Walcott, of this survey, at 

 length found some limestone bands near the base of Powell's Shinarump, 

 which seem to establish pretty conclusively their Permian age. Bnt 

 the fossils so far discovered have only a small vertical range, and lie 

 near the base of the group. Above them are many hundred feet of 

 beds which yield no fossils at all. While some of them are unquestion- 

 ably Permian, it still remains to find the horizon where the Permian 

 ends and the Trias begins. The Trias is as destitute of fossils as the 

 Permian, excepting, however, some which are useless for determining 

 age. In cases like this the geologist finds himself in trouble. Ho is 

 quite sure that he has beds of two distinct ages; and he must, for pur- 

 poses of discussion, separate them somehow; if not by a natural and 

 unmistakable dividing horizon, then by an arbitrary and provisional 

 one, subject to amendment by future research. But he must look very 

 carefully for a natural horizon of separation. His course of procedure 

 would be somewhat as follows. Starting, for instance, with those strata 

 which he was sure were Triassic, he would examine the beds downwards 

 and finding no fossils would pay attention to their lithological characters. 

 Finding no marked difference in the beds, and finding a strict parallelism 

 or "conformity" in the several members, he would infer that: they were 

 deposited under conditions which were substantially identical throughout 

 the period of deposition. But if he at length reached a stratum of very 

 different character, say, for instance, after passing down through a 

 great series of sandstones and shales, he came to a heavy mass of lime- 

 stone or a bulky conglomerate, he would have found at last a " break" 

 in the continuity or homogeneity of the group. Here, at last, is some- 

 thing which he can use. It may or it may not be synchronous with 

 the dividing horizon used in Russia, England, or Kansas, but it is at 

 all events not far from it; and it is something palpable, distinct, and 

 recognizable by those who come after him. In this way Mr. Walcott 



*For tbo information of the general reader it maybe explained that when the 

 geologist entering a new region discovers a well-defined group of beds which either 

 yield no fossils at all or yield such as do not enable him to determine conclusively the 

 age of the series, he does not assign the beds to any definite age or system, but gives 

 them a purely provisional and local name which is dropped as soon as the true age is 

 established. 



