Report on the Geology of the High Plateaus of Utah, with Atlas

Page 240

240 GEOLOGY OF THE HIGH PLATEAUS. Section II.â€"Monroe Amphitheatee. Beginning at the verge of the upper amphitheater and descending west-southwest; altitude, about 10,100 feet. Feet. 1. Argilloid trachyte, reddish brown, with large orthoclase crystals.......... 27 2. Granitoid trachyte, very coarse and somewhat hornblendic, three layers and probably more................................................... 100 3. Fine-grained dolerite..................................................- 13 4. Fine-grained dolerite, perhaps two layers..................... .......... 23 5. Hornblendic trachyte, rather fine grain................................... 80 6. Granitoid trachyte.................... ................................ 45 7. Light red trachyte, brick-like texture................................... 30 8. Argilloid trachyte, light gray, with small crystals and grains of magnetite, and probably six or seven layers.................................... 220 9. Augitic andesite, very massive and in many sheets....................... 190 10. Hornblendic trachyte.................................-................ 40 11. Granitoid trachyte, coarse grain........................................ 175 12. Dolerite............................................................. 20 13. Granitoid trachyte, unknown thickness. Section III.â€"Monroe Amphitheater. Beginning near the base of the great upper cliff on the northern side of the amphitheatre and descending south-southwest; altitude, about 9,800 feet. Feet. 1. Granitoid trachyte, light reddish-brown, with crystals of magnetite........ 38 2. Granitoid trachyte, light gray, coarser than the foregoing, containing mag- netite ........................................................... 65 3. Argilloid trachyte, very heavy masses, probably several layers but divis- ional lines not readily made out, dark-colored porphyrinic crystals, much weathered on all surfaces........................-.................. 230 4. Dolerite, large plagioclase crystals, dark-gray color.................... 40 5. Augitic trachyte (?), several layers..............................,........ 70 6. Hornblendic trachyte (!)............................................... 150 7. Argilloid trachyte, light reddish color.................................. 115 8. Augitic trachyte....................................................... 30 9. Dolerite............................................................... 50 10. Granitoid trachyte, slightly hornblendic, several layers, not readily separable.............................................................. 200