If the structure of the mountains be as novel to the reader as it was to the writer, and if it be as strongly opposed to his preconception of the manner in which igneous mountains are constituted, he may well question the conclusions in regard to it while they are unsustained by proof. I can only beg him to suspend his judgment until the whole case shall have been presented. On some accounts it would have been well to follow in writing the order of investigation, and develop the general plan of structure as it was developed in the field, by the addition here of one element and there of another; or at least to assemble the facts before announcing my deductions. But such a course would be at the expense of an important element of convenience and brevity. As will appear in the sequel, the preliminary explanation of the type structure furnishes a complement of categories and terms by the aid of which the description of the details of observation, essentially tedious, is greatly abbreviated. [Illustration: FIG. 7.—Ideal Cross-section of a Mountain of Eruption. ] [Illustration: FIG. 8.—Ideal Cross-section of a Laccolite, showing the typical form and the arching of the overlying strata. ] It is usual for igneous rocks to ascend to the surface of the earth, and there issue forth and build up mountains or hills by successive eruptions. The molten matter starting from some region of unknown depth passes through all superincumbent rock-beds, and piles itself up on the uppermost bed. The lava of the Henry Mountains behaved differently. Instead of rising through all the beds of the earth’s crust, it stopped at a lower horizon, insinuated itself between two strata, and opened for itself a chamber by lifting all the superior beds. In this chamber it congealed, forming a massive body of trap. For this body the name _laccolite_ (λάκκος, _cistern_, and λίθος, _stone_) will be used. Figure 7 and Figure 8 are ideal sections of a mountain of eruption and of a laccolite. The laccolite is the chief element of the type of structure exemplified in the Henry Mountains. It is evident that the intrusion of a laccolite will produce upon the surface as great a hill as the extrusion of the same quantity of matter, the mass which is carried above the original surface being precisely equivalent to that which is displaced by the laccolite; and it is further evident that where the superior rock is horizontally stratified every stratum above the laccolite will be uplifted, and, unless it is fractured, will be upbent, and will portray, more or less faithfully, by its curvature, the form of the body it covers. Associated with the laccolites of the Henry Mountains are _sheets_ and _dikes_.