A third feature of the diagram is the restriction of the upper zone of laccolites to the northward clusters; Mounts Ellsworth and Holmes contain laccolites of the lower zone only. This fact of distribution is correlated with a fact of denudation—namely, that in the general degradation of the country, the region about the southern mountains has lost two thousand feet more than the northern. One fact is probably the cause of the other. The absence of laccolites of the upper zone at the south may have permitted the greater degradation; or the greater degradation may have caused the destruction of several of the upper laccolites. The fact that the difference in degradation can be independently accounted for is favorable to the latter supposition. The Colorado River which is the main artery of drainage for the whole region, flows close to the bases of the southern mountains, and the rapid declivity from the mountain summits to the river has given and still gives exceptionally great power to the agents of erosion. No other cause is needed to explain the difference of degradation, and the absence of laccolites of the upper zone is explicable without assuming that they were never present. The negative objection to the idea that the southern mountains originally possessed laccolites of the upper zone being thus disposed of, it is worth while to inquire whether there is any evidence in its favor. If superior laccolites existed, they would be sure to leave behind them a record of the conduits through which their lava was injected. A dike or a chimney must always connect a laccolite with the source of its material; and the removal of the laccolite necessarily exposes a cross-section of its stem. The discovery of such a dike can be regarded, not indeed as a proof of the former existence of a superior laccolite, but as demonstrating its possibility. The summits and flanks of Mounts Holmes and Ellsworth bear many dikes, which have been regarded as subsidiary features of the laccolites beneath them, but it is quite possible that any one of them formerly led to another laccolite above. The upper laccolite may have been first formed and then have been lifted, dike and all, when the lower was intruded; or it may have been last formed, and been fed through a fissure which traversed the lower after its congelation. The only dike which was discovered in the vicinity of these mountains, without being upon them, stands midway between them. It is the only observed dike of the Henry Mountains (excepting always the alpine district of Mount Ellen), which is not so closely associated with some laccolite as to seem an accessory feature, and its exceptional position has led to the suspicion that it belonged to an overlying laccolite.