BASALT FIELDS OF THE MAEKAGUOT. 201 



the tributary ravines of Mammoth Creek (the main fork of the Sevier 

 River), and reach a point about 6 miles from their origin. 



Besides this field of very recent basalt, remains of much more ancient 

 basalt are found in the vicinity and in much larger amount. In truth, the 

 basaltic eruptions go back to a period sufficiently remote to have permitted 

 important changes in the configuration of the country to take place in the 

 interval separating the present from the earliest eruptions of this class. 

 During that interval a considerable number of outbreaks, separated by 

 many centuries (pi*obably hundreds of centuries), have occurred. Basalt 

 fields of different ages are readily distinguished. Among the oldest, proba- 

 bly, are the first basalts spoken of in this chapter. Of an antiquity which 

 may be quite as great are two large masses, lying respectively southeast 

 and southwest of Pan quitch Lake. The southwest field is much eroded, 

 and consists of a tabular mountainous mass immediately overlooking the 

 very recent basalt field just spoken of. The edges of the sheets composing 

 this tabular mass project in bold cliffs around its flat summit in the same 

 manner as is frequently seen in lower regions, where buttes of sedimentary 

 rocks owe their origin and preservation to a protecting mantle of lava. On 

 all sides it is girt about by a talus of blocks, which have fallen by the sap- 

 ping of the foundations of the mass through untold ages. Since this lava 

 was disgorged broad valleys and deep ravines have been scored in the plat- 

 form of the Markagunt, and the minor details of topography arising from 

 the general process of surface sculpture have been carved out, and an 

 older topography has been swept away or so completely remodeled that 

 it cannot now be reconstructed. 



Southeast of the lake a wide expanse of country has been covered 

 with ancient basalt, but only remnants are now left, covering mesas and 

 buttes of sedimentary rocks and overlying fields of still older trachytes and 

 volcanic conglomerates. Ravines of considerable magnitude and broad 

 vallej^s have been cut into the country which they once covered, and these 

 excavations have in several instances given passage to more recent floods 

 of basalt, some of which extend as far east as the Sevier River. These 

 later basalt fields are in an excellent state of preservation, but soil has 

 accumulated upon them, and the face of the rocks shows deep weathering. 



