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Show BULLION CASOK 175 their existence by the recurrence of beds, which seem to be identical both individually and serially, at higher and higher levels up the canon. As we ascend Bullion Canon from Marysvale we observe on either side a hard quartzitie rock well bedded in massive layers, exhibiting considerable metamorphism. It is also somewhat variable in the dip. The strata incline upward at first, but soon flex easily back until the dip is westward. The thickness of the series seems to be very considerable, though the apparent thickness may be partly due to repetitive faults of small shear. At a distance of about 3 miles from Marysvale and 2,600 feet above that village, we come upon the volcanic series. A mass of dark-colored liparite rests upon the quartzite,, having a thickness of about 450 feet About 500 feet higher the quartzite reappears, being probably the same bed as below, but thrown up by a minor displacement, and it is covered by the same or a similar sheet of liparite. The quartzite, however, is more altered than the portion of it below, and in general as we ascend from Marysvale through the quartzitie beds the signs of increasing alteration are unmistakable. From this point upwards eruptive rocks alone are seen. The lower masses are dark liparites, with abundant quartz and monoclinic feldspar and decomposed hornblende. Still higher rocks of a porphyritic texture and a dark purplish hue lie in great volume. They have a striking resemblance superficially to the argilloid trachytes of the central and eastern plateaus, but contain abundant quartz, and the microscope confirms their rhyolitic character. These two groups of eruptions are separated by local conglomerates derived from the older of them, and the surface of the latter is seen to have been much eroded, indicating a considerable interval of time between the periods of activity. The summit of the series consists of a group of rhyolites (proper rhyolite), which contrast strongly with those beneath. They are very light colored, without crystals, and yet not hyaline. They are highly siliceous, and exhibit in the thin sections a fibrolitic or spherolitic groundmass of beautiful texture and very interesting. Some of the specimens are exceedingly siliceous, and are resolved under the microscope into an aggregation resembling very fine-grained quartzite and appear to be quite abnormal. The light-colored masses are generally true rhyolites of no uncommon kind. This rock forms the lofty peaks crown- |
