From this it appears that the laccolites of the upper zone, extending from the lower part of the Blue Gate Shale to the upper part of the Flaming Gorge Shale, bore loads of which the mean densities were from 2.31 to 2.34, and that laccolites of the lower zone, which has its upper limit in the Shinarump Shale, bore loads of which the mean densities were 2.32 and upward. If the positions of the laccolites were determined purely by the law of hydrostatic equilibrium, then these figures define the density of the molten trachyte, and show that its contraction in cooling—from the density 2.34 to the density 2.61—was about one-tenth of its volume. THE STRETCHING OF STRATA. It has been the opinion, not only of the writer but of other students of the displacements of the West, that the ordinary sedimentary rocks, sandstone, limestone, and shale, are frequently _elongated_ as well as compressed by orographic movements, and that this takes place without any appreciable metamorphism; but it is difficult to find opportunity for the demonstration of the phenomenon by measurement. When a fold is made in a level stratum, either of two things may take place; the portions of the stratum which remain level at the sides may approach each other; or the stratum may be stretched. But when a circular portion of a continuous level stratum is lifted into a quaquaversal arch (as illustrated in Figure 11), an approach of the level portions is out of the question, and there must be a stretching or a fracture. Of the unfractured quaquaversals of the Henry Mountains there is one which combines all the essentials of a crucial case. The Lesser Holmes arch is nearly isolated; on three sides it rises from the undisturbed plateau, and on the fourth it joins a similar but fractured dome. The major part of its surface is composed of one bed, the Vermilion Cliff sandstone, broken only by erosion. Comparing the length of this bed in its present curved form with the space it must have occupied before it was upbent, I find that in a distance of three miles it has been elongated three hundred feet. Moreover there is every reason to suppose that the elongation was produced quickly, or at least by a succession of finite rather than infinitesimal increments; for the lifting of the arch was caused by the intrusion of a laccolite, and though the latter may have been built by the addition of many separate lava flows, it could not have risen with secular and continuous slowness. The molten trachyte, rising through a passage and into a reservoir that were comparatively cool, would have clogged itself by congelation had it not moved with a certain degree of rapidity. [Illustration: FIG. 52.—Cross-section of an uplifted dome. The dotted lines show the original position of a bed; the curved lines, the imposed. ]