“There is one specimen (No. 57, from dike in Shinarump shale, Mount Ellsworth) which constitutes an exception to the foregoing. By inspection of a hand specimen it might hastily pass for a very compact andesite, but the observer will be instantly undeceived by applying the microscope. It consists of imperfect crystals, which must be sanidin, imbedded in a very close groundmass composed of a material which differs from the foregoing trachytes in being wholly amorphous. Between crossed Nicols the paste transmits no light whatever, though between the parallel Nicols it closely resembles the others. The hand specimen is very dark colored (gray), but the slide is sufficiently translucent. The felspar crystals are either fragmental or very imperfectly developed as to their edges and angles, but polarize very sharply. The amount of twinning is very small, but it appears occasionally. The rock is undoubtedly a trachyte, but an unusual one. Its dark color is not due to hornblende nor to magnetite, both of which occur in it very sparingly, especially the former. “I have already remarked that the only frequent minerals besides felspar are hornblende and magnetite. Apatite occurs and is tolerably plentiful in a few of the specimens, but absent from most of them. The crystals are all small, requiring a low power for the determination of the larger and a high power for the smaller. They present no peculiarities. Of very rare occurrence is nepheline. This mineral is usually associated with the more basic volcanic rocks and seldom penetrates the trachytic group. Quartz is almost equally rare. The absence of any great variety in the mineral species is quite normal, except possibly the total absence of mica, which is usually present and frequently the only associate of felspar in the western trachytes. The Henry Mountain rocks do not so far as I can discover contain a trace of it. “In answer to your particular inquiries— “1st. ‘Is the paste vesicular, or is there any evidence in the crystals to indicate the pressure under which they were formed?’ I can only say that the paste is extremely compact and contains no vesicles even in those specimens of which the aspect is most decidedly trachytic. Some of the larger crystals of felspar contain an abundance of pores or vesicles which may have contained liquids, but with a ¼-inch objective they are too small for treatment by the method you refer to. A few large cavities, usually of irregular shape, occur, but I am quite unfamiliar with the practical treatment of this subject and cannot advise you. I do not find any cavities still containing fluids, and I presume that even if they existed they would be difficult if not impossible to gauge on account of the impellucidity of the felspar. I presume quartz is the most favorable mineral for this investigation on account of its transparency and the greater frequency of its large cavities. Quartz however is almost the scarcest of the contents of these rocks.