EPOCHS OF ERUPTION— EHYOLITES AND BASALTS. 61 



but each preserving certain dominant features. The older of the two has 

 the character of hparite — 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 colored — 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 indi- 

 vidual 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 Markiigunt, and a somewhat smaller basaltic area is found in the mid- 

 dle 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 cen- 

 tral 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 dbzen other streams, in 

 various localities, might be named of which the antiquity can hardly exceed 



