They were not formed at any late stage of the final erosion, for they conserve tables along their western base, which but for their shelter would long since have disappeared. From the end of the Cretaceous period to the end of the desiccation of the basin there is no event with which the laccolites can be directly connected. There is however a consideration which in an indirect way sanctions the opinion that the epoch of igneous activity was after the deposition of the Tertiaries and before their erosion. The Masuk Sandstone is at once the summit of the Cretaceous and the highest bed in the present Henry Mountain section. If it were restored over the entire range, the laccolites of the upper zone would have on the average thirty-five hundred feet of cover, and those of the lower zone nearly seven thousand feet. This was the depth of their original cover, if they were intruded at the close of the Cretaceous age. During the epoch of Tertiary deposition and the subsequent epoch of erosion, the cover first increased in depth and then diminished, having its maximum at the end of the Tertiary deposition. If it can be shown that the original cover of the upper laccolites exceeded thirty-five hundred feet, the question of age will be reduced to comparatively narrow limits. In order to discuss the problem of the original depth of cover it will be necessary to consider another matter, of which the connection will not at first be apparent. _The size of laccolites._—It is a matter worthy of note that no laccolite of inconsiderable extent is known in the Henry Mountains. The smallest which has been measured is more than half a mile in diameter, and the largest about four miles. The phenomenon does not occur upon a small scale, but has a definite inferior limit to its magnitude. Let us seek an explanation of this limit. The dome of strata which covers a laccolite has for its profile on every side a monoclinal curve. In Figure 52 the section of a dome exhibits a monoclinal flexure in _s a_ and again in _s b_; and the dome being approximately circular this flexure completely surrounds it. We may even describe or define the dome as a monoclinal flexure encircling a point or a space. Considering now that when the laccolite was injected the overlying strata were lifted, and that this disturbance was communicated upward to the then existing surface of the earth, we may properly speak of the lifted body of rock as a cylinder bounded on every side by a monoclinal flexure. Furthermore, since the monoclinal flexure is the structural equivalent of the fault[4], we may render our conception still simpler by replacing in imagination the encircling flexure by an encircling fault, and picturing to ourselves the uplifted rock mass as a simple cylinder, perfectly divided from the surrounding rock and slidden upward so as to project above the surface an amount equal to the depth of the laccolite. Footnote 4: