If however all of these experiments were trustworthy and their results were concordant, their bearing upon the problem of the laccolites would still be slight. It is generally conceded that the fusion of lavas is hydrothermal, while in all the experiments recourse was had to dry fusion; and the densities attained in the two ways are necessarily different. The practical difficulty in the way of restoring the natural molten condition is great and may be insuperable, but unless it shall be overcome we cannot learn experimentally the changes of density which igneous rocks undergo in congelation. There is a fact of observation which tends to sustain the view that the laccolitic rocks contracted less in cooling than the volcanic. The prismatic structure is produced by the contraction of cooling rocks during and after solidification. That it does not occur in the Henry Mountain trachytes indicates that their contraction was small. That it does occur at numerous localities in Utah in basalts, indicates that their contraction was relatively great. Mr. Jukes, in his Manual of Geology, says that it is most frequently exhibited in “doleritic lavas and traps, being especially characteristic of basalt, but occurs almost as perfectly in some greenstones and felstones”; and in the range of my own observation I can recall no instance of its occurrence in other than basic rocks. For the sake of comparing the densities of the intrusive rocks with those of the strata which contain them, a number of determinations were made of the specific gravities of specimens representative of the trachytes and of the several sedimentary groups of the Henry Mountains. Trachytes were selected to represent as great a variety of locality and relation as possible, and at the same time exclude all specimens which showed traces of decomposition. Hand specimens weighing from one hundred to four hundred grains were used, and these were weighed first dry, and then suspended in water. By using such large quantities averages were obtained of a rock which, minutely considered, is heterogeneous; and by using the blocks entire instead of pulverized or granulated, the state of aggregation of its minerals was included as an element of the specific gravity of the rock. It will be observed that the range, 2.54 to 2.66, is very small.