Page:Popular Science Monthly Volume 40.djvu/244

232 is unconformably overlain by the Cretaceous strata of the coastal plain, proving that the sandstones were not only tilted but deeply eroded before the Cretaceous beds were laid upon them. The formations in New Jersey and Connecticut are so much alike that we may safely conclude that the period of dislocation was the same in both; hence we shall suppose that the Meriden sandstones and lava-sheets were tilted and faulted into the position illustrated in Fig. 10 during the interval between Triassic and Cretaceous time—that is, in the Jurassic period. From that time to now their history is concerned chiefly with the erosion by which their original constructional inclined planes have been reduced to their present surface of varied topography.

There is good reason to think that the history of the erosion is a double one, comprehending first a longer cycle, and second a shorter cycle of time. During the first cycle, the great relief of the uptilted beds was reduced to a lowland of denudation, a surface of a moderate relief close to the base-level of erosion, an almost plane surface, a "peneplain"—the evidence of this being found in the even uplands of the crystalline plateaus which now inclose the Triassic valley on the east and west. No explanation for the evenness of these plateaus can be found save the one which regards them as having been reduced from some greater mass by a long-continued process of erosion, at a time when the region stood somewhat lower than now—low enough to place the present plateau-like uplands close to sea-level; and the sandstones, shales, and lava-sheets between the two plateaus undoubtedly suffered the same denudation. This is indicated in Fig. 11, in which all the upper part of the model as shown in Fig. 10 has been removed; the obliquely beveled surface of the beds now represents the lowland of denudation, or peneplain, to which they were reduced. The effect of the oblique faulting is now rendered apparent by the dislocations in the belts of the different outcrops. The main sheet of lava, for example, is seen in each of the blocks into which the formation is divided by the faults; so is the belt of shales lying under it, and so on with every member of the series.