OCR Text |
Show SUCCESSION OF EEUPTIONSâ€"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 scaling 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 doubtful 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 faulting 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 southwest; 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............................................................ 280 2. Coarse dolerite, several layers............................................ 60 8. Somewhat finer dolerite, but with well-marked porphyritic plagioclase....... 35 4. Argilloid trachyte, reddish brown....................................... 140 5. Gray granitoid trachyte.................................................. 40 6. Dolerite, very fine-grained and compact.................................. 12 7. Argilloid trachyte, several layers........................................ 110 8. Very coarse and porphyritic dolerite, dark gray, many layers................ 85 9. Granitoid trachytes, several layers, thickness unknown; only 60 feet meas- ured ................................ ...........................___ 60 |