found on pages 61 to 71 and 163 to 168 of the Annual Report for 1874 of the “Geological and Geographical Survey of the Territories.” If we turn now to the distinctively volcanic mountains of the Plateau province—to those which are built by eruption at the surface—we leave at once the porphyritic trachytes. Mount San Francisco, Mount Bill Williams, Mount Sitgreaves, Mount Kendrick, Mount Floyd and the Sierra Blanca (of Arizona) are all composed of basic trachytes, and so are the Aquarius Plateau and the many tables that lie beyond it. The Mogollon group, the Marcou Buttes, the minor cones about Mount San Francisco, the Uinkarets, and the Panguitch Lake group are basaltic. In each of these instances the igneous rock issued above the surface and there is no evidence by displacement that any portion of it was deposited below. Mount Taylor may be an exception. In the character of its lava and its general features it resembles Mount San Francisco, but there are disturbed strata on its southern flank, and it is possible the mountain is both extrusive and intrusive. Extrusion and intrusion are probably combined in some small tables lying fifty miles north of the Henry Mountains. They are built of Flaming Gorge shale, preserved from erosion by dikes, sheets, and (probably) outflows of basalt. Combining all these facts we attain to a simple relation between two types of igneous rock on the one hand, and two types of mountain structure on the other. One type of rock is acidic, including “porphyritic trachyte” and “eruptive granite”, and its occurrence is without exception intrusive. The other type of rock is basic, including basic trachyte and basalt, and its occurrence is almost uniformly extrusive. It is not possible to combine the two groups of phenomena by saying that in one case the eruptive cones cover laccolites, and in the other the laccolites have been covered by eruptive cones which have disappeared; first, because many of the eruptive cones are too well exposed to admit of the concealment of laccolitic arches beneath them; second, because the two types of lava are essentially different. The acidic type if extruded at the surface would be an ordinary trachyte; the basic type if crystallized under pressure would be classed with the greenstones.