“The dominant felspar is orthoclase, but a portion of it is triclinic, and I presume this portion is albite, with an occasional occurrence of oligoclase. The groundmass in which the crystals are included is in most cases decidedly compact and without distinguishable crystals, but shows between the crossed Nicols closely aggregated luminous points, which with a ¼-inch objective are resolved indistinctly into felspar. In some cases the crystals of the groundmass are quite apparent with an inch objective, and their species determinable. But the greater portion of the groundmass is quite amorphous, and does not polarize light at all. The proportion of crystalline to amorphous matter in the paste is highly variable—in some cases it is quite bright between crossed Nicols, in others far less so, and in none is it entirely dark. Those specimens which have the finer and more amorphous groundmass have the larger and more perfect crystals of felspar—an association of properties which is not wholly without qualification, but still sufficiently decided. “Turning to the included felspars, their mode of occurrence is quite an uncommon one, I believe, so far as the eruptive rocks of the Rocky Mountain region are concerned, and give rise to some hesitation before assigning them definitely to the trachytic group. In a great many cases the felspathic crystals are well developed, and so large and so nearly perfect that their aspect is decidedly porphyritic. This is especially the case with the dikes of Mounts Ellsworth and Holmes (Nos. 56, 61, 68, and 69). The orthoclase is invariably of the white “milky” variety, with the exception of a single specimen from a Mount Ellsworth dike (No. 57), where it is present as sanidin. (This is an exceptional rock in all respects, and will be spoken of hereafter.) Nearly all of them appear to have been subject to alteration by chemical action since their formation as is indicated by their diminished power to polarize light. Whether this is due to atmospheric weathering or to changes _en masse_ it is of course impossible to distinguish with certainty, though I incline to the latter view since it is manifested as decidedly in specimens which show no external indications of weathering as in those which do show them. It is not uncommon to find crystals which have almost entirely ceased to polarize. The zonal arrangement is very common in the crystals, and some of the zones contain numberless minute fluid cavities in the largest crystals. The foreign substances included in the crystals present no novelty, being the ordinary films of hornblende, minute needles of felspar, granules of magnetite, and those dust-like points of brownish yellow color which are the proper inclusives of the groundmass.