button.] THE PERMIAN. ^3 



seized upon the Shinarump conglomerate as a divisional stratum between 

 the Trias and Permian. But another perplexing- question arose. To 

 which of the two series should the conglomerate itself be assigned ? 

 And the question is not at first an easy one to answer. Immediately 

 above it is a series of sandy shales such as beggar description on account 

 of their gorgeous colors. Immediately below it is another series of 

 sandy shales so similar to the one above that we never know which of 

 the two we are looking at unless the conglomerate is in sight as a 

 "benchmark." Mr. Walcott settled the question (provisionally, of 

 course) in the following way. The summit of the lower series shows 

 in many places that immediately after it was deposited it was slightly 

 eroded, and the contact of the conglomerate shows an "unconformity 

 by erosion." The contact of the conglomerate with tin- upper series 

 shows no such unconformity. Xow, an unconformity means to the 

 geologist a break in the continuity of deposition, and in the absence of 

 reasons to the contrary, and with no better divisional criterion at hand, 

 it may be used to separate two series of beds, lie therefore assigned 

 the conglomerate to the Trias, and the beds below he placed in the 

 Permian. 



Mr. Walcott's conclusion is no doubt the best which can be reached 

 with our present knowledge, but it is very inconvenient and awkward to 

 the geologist who is required to map the, distribution of the strata and 

 their topographical features. In all of the other formations each group 

 forms its own terrace or series of terraces. As we descend them we find 

 ourselves, when we reach the foot of the Eocene cliff, upon the summit 

 of the Cretaceous. Peaching the foot of the Cretaceous cliffs, or slopes. 

 we are upon the broad expanse of the Jurassic platform. Descending 

 the Jurassic, we find the Trias coming out from the base of the Jurassic 

 Cliffs; but when we descend the Vermilion Cliffs, we have not reached 

 the Permian. The Trias is still beneath us, poshing out its basal mem- 

 ber, the Shinarump conglomerate, clear to the cresMineof the Permian 

 wall. In the Jurassic terrace and in its terminal cliff we find none but 

 Jurassic strata. Similarly, also, on the cliffs and terrace platforms of the 

 Cretaceous and Eocene; but the Permian terrace is everywhere sheeted 

 over with a solitary stratum of the Trias. Somehow we cannot help 

 thinking that the conglomerate has no business there, and that it ought 

 to have been cut off at the base of the Vermilion Cliffs, or else it ought 

 to be relegated to the Permian. In delineating the distribution of 

 the formations by means of colors on the map, the ordinary practice 

 would require us to extend the Trias to the brink of the Permian Cliffs, 

 for in such delineations we only profess to show the surface exposures 

 of the several groups ; but this would confound the Permian terrace with 

 the Trias, and obliterate the individuality of the former, whereas in the 

 topography both are as distinct as land. and water. To preserve this 

 distinction the Shinarump is denoted by a special modification of the 



