| OCR Text |
Show EPOCHS OP EBUPTIONâ€"EHYOLITES AND BASALTS. 61 but each preserving certain dominant features. The older of the two has the character of lipariteâ€"a porphyritic texture with conspicuous crystals of feldspar and quartz, and having a superficial resemblance to some common trachytes, but more glassy or hyaline. They are usually very dark colored. The later varieties are nearly white or cream coloredâ€"sometimes ashy-gray, without any apparent crystals even under the microscope, but showing a reticulated or globulitic ground-mass of great beauty and interest. The rhyolites of the Markagunt have a superficial resemblance to trachyte, being dark gray and porphyritic, with a texture which is decidedly trachy-tic, but the abundance of free quartz and the fluidal aspect of the ground-mass under the microscope reveal its true affinities unmistakably. Upon the western verge of this plateau they have piled up some lofty masses with broad tabular summits. They are seen in many places to rest upon older trachytes and in others are overlaid by basalt. The basaltic eruptions were very numerous throughout the district, but never attained the magnitudes seen in the other groups. Most of the individual coulees are relatively small. The largest masses are seen on the southwestern flank of the Tushar. Here numerous eruptions from the same vents have piled up nearly a thousand feet of basalt and spread the lava confusedly over a considerable area. A large field, with many cones still standing in a dilapidated condition, is found at the extreme southern portion of the Markagunt, and a somewhat smaller basaltic area is found in the middle of that plateau. In every case true basalt is here the youngest of the eruptive rocks, but much of it still shows considerable antiquity. In the Tushar the larger vents have been so far obliterated that the cones have vanished and left the determination of the sources of the lavas to other characters. In the central part of the Markagunt the cones have nearly faded away, but are still recognizable. On the other hand, some of the basalts are strikingly recent, and a few so fresh that no appreciable change has taken place since their orifices became silent. Just south of Panquitch Lake, in the Markagunt, are a number of streams, which, so far as appearance is concerned, might have been erupted less than a century ago. Half a dozen other streams, in various localities, might be named of which the antiquity can hardly exceed |
