| OCR Text |
Show SUCCESSION OF ERUPTIONS IN THE MARKAGUNT. 197 rough surface ; but this trachyte breaks with an exceedingly jagged, angular, and irregular fracture, so that it is impossible to hammer out a neat and shapely specimen. The grandest masses of trachyte, not only in the Marka-gunt but in the other plateaus, consist of this variety. It lies in immense beds, often two or three hundred feet in thickness, spreading out over many square miles with remarkable regularity and homogeneity. In the Marka-gunt it forms mesa-like platforms, ending in low precipices, wdiere the shallow canons and ravines have cut into it. It breaks up or rather crumbles with unusual facility for an eruptive rock, producing a coarse gravel, which floors the ravines below. This rock is so distinct in its characters that it seems almost to justify a separate name, but I shall content myself with a purely descriptive designation, and call it argilloid trachyte. The augitic varieties of trachyte are found in sheets, which are usually much thinner and cover smaller areas, though the number of them is much greater. The total bulk is less than that of the argilloid variety, though absolutely it is very great. The rhyolites are the third group of eruptives found in the Markagunt. They are seen in large masses along the very highest part of the plateau, from the crest of which they poured out in massive sheets. They are probably as ancient as the older liparitic masses of the Tushar, but always overlie the trachytes whenever they are in contact with them. They belong altogether to the liparitic sub-group, with an abundance of porphyritic crystals of feldspar and quartz. None of those hyaline fluent rhyolites which characterize the northern Tushar are seen here. Although their volume is very great, it is far less than that of the trachytes, and the areas which they cover are much smaller. The fourth group is the basaltic. Among the High Plateaus the Markagunt and Tushar alone present extensive outpours of rocks of this class. A few small eruptions are found in the eastern plateaus and notably in the intervening valleys, but they are not comparable in extent to those of the Markagunt. Here they are confined to the southern half of the plateau. A little south of the center is a large tract in which are still preserved remnants of a considerable number of basaltic craters, though so much degraded that they are not immediately recognized. They form a large |
