Perhaps the most striking feature of this formation of sandstone, is the great and abrupt change in the inclination of the strata in the parts near the granite. We have already described this in a manner sufficiently explicit, as we suppose, to convince most of our readers that since the deposition of the sandstones, a signal change must have happened in the elevation of the secondary aggregates as compared with the granite. The appearances are precisely {289} such as we must suppose would have ensued, had the sudden emerging of the granite broken off, and thrown into an inclined or vertical position the margin of the horizontally stratified rocks of the plains. We are conscious that inclined strata of sandstone are by no means infrequent about the declivities of lofty mountains, but we are not well assured that the same strata being traced to a little distance, are often found in a horizontal position in the plains, as is the case in the instance under consideration. It may perhaps be thought possible that the gradual wearing away, by the agency of rivers, of some portions of the sandstone, may have been sufficiently extensive to have occasioned that change of elevation of which we speak; and that those rocks now found in an inclined position, are insulated portions of what was formerly the upper part of the stratum, which having been undermined on their eastern side, and supported by the granite on their western, have fallen into their present situation. This supposition, however, seems incompatible with the vast magnitude and extent of these rocks, and entirely irreconcilable to the fact that they dip to a great and indefinite extent below the present level of any of the beds of the river. The position of this formation in relation to the granite is similar to that of the sandstone of Guachaco, in South America, observed by Humboldt; also to that spoken of by Mr. Burkhardt, at the entrance of Nubia, superimposed upon the granite of Syene, and to that mentioned by Mr. Schoolcraft, as found near Lake Superior, but it does not appear that those formations have the same peculiarities in regard to inclination. FLETZ TRAP ROCKS Another family of rocks, of recent formation, and connected with the sandstone last mentioned, remains to be noticed.