FIG. 1.—Fossil Suncracks in the Shinarump Shale. ] From the base of the Vermilion to the summit of the Cretaceous no evidence of land erosion has been found; but the association of coal seams with all of the Cretaceous sandstones except the lowest, shows that the sea-bottom was frequently brought to the surface of the water if it was not carried above. Thus it is evident that the strata of the Henry Mountain region do not represent continuous sedimentation. At the close of the Aubrey epoch, in the middle of the Shinarump, and again at the close of the Shinarump, not merely was the accumulation of sediments interrupted, but the process was reversed, and a portion of the deposits which had already been formed were excavated by the agency of rains and rivers, and swept away to some other region. Each break is indefinite, alike as regards the interval during which the record of the sea was interrupted and as regards the extent of the record which was at the same time obliterated. And yet the evidence of these breaks is of such nature that it would probably elude observation if a single section only were examined, and in a region masked by the soil and vegetation of a humid climate it would hardly be discovered except by accident. The parallelism of contiguous strata is not alone sufficient evidence that they were consecutive in time. At the close of the Cretaceous period there came an epoch of disturbance. The system of strata which has been described was bent into great waves, and the crests of the waves were lifted so high above the sea that they lost thousands of feet by erosion. In the troughs between the waves lakes remained, in which the material removed from the crests was redeposited, and by a later change the lake waters rose so as to cover the truncated crests, and deposit upon the worn edges of the upbent strata a series of unconforming, fresh-water, Tertiary sediments. Thus was produced the only _unconformity of dip_ which involves the Henry Mountain rocks, and even this is not to be observed in the immediate vicinity of the mountains, for a later erosion has thence removed all of the Tertiary strata, and has resumed the degradation of the older beds. THE GREAT FOLDS.