SUCCESSION OF EEUPTIONS— SECTIONS. 239 



SUCCESSION OF ERUPTIONS IN THE SEVIER PLATEAU. 



The following successions of volcanic beds were observed in the Sevier 

 Plateau. An effort was made to obtain some good sections in the Monroe 

 amphitheater, but proved unsuccessful, partly owing to the difficulty of scal- 

 ing the rock faces and penetrating the clefts, and partly to the fact that the 

 chaotic condition of the rocks in many places makes the section of doubt- 

 ful value. Thus lavas of later age, filling ravines scoured in older floods, 

 occupy lower positions than the latter, and the contacts are lateral instead 

 of by superposition. Some present thick lenticular outcrops, some recur 

 (probably) at different altitudes. There is much local shattering and fault- 

 ing which cannot be restored, and many masses vary so much in thickness 

 that it would be misleading to state it without qualification. Most of the 

 heavy masses are presumed to consist of several distinct coulees, but the 

 separation is rarely visible or accessible. These difficulties and many 

 others increase towards the base of the series and are troublesome near the 

 summit. The chief value of a collection of sections is the illustration it 

 furnishes of the secular order of eruptions of the various groups of rocks 

 and their intercalary character. 



Section I. 



Commencing at the summit of Mount Thurber and descending south- 

 west; altitude, about 11,160 feet. 



Feet. 



1. Granitoid trachyte, composed of layers, ranging from 30 to 80 feet in thickness, 



the number of which is unknown, and varying but little in lithological 



character 2S0 



2. Coarse dolerite, several layers. 60 



3. Somewhat finer dolerite, but with well-marked porphyritic plagioclase. 35 



4. Argilloid trachyte, reddish brown 140 



5. Gray granitoid trachyte 40 



0. Dolerite, very fine-grained and compact 12 



7. Argilloid trachyte, several layers 110 



8. Very coarse and porphyritic dolerite, dark gray, many layers S5 



9. Granitoid trachytes, several layers, thickness unknown; only 60 feet meas- 



ured 60 



