240 GEOLOGY OF THE HIGH PLATEAUS. 



Section II. — Monroe Amphitheater. 



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. Finegrained dolerite 13 



4. Fine-grained dolerite, perhaps two layers 23 



5. Hornblendic trachyte, rather flue 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 — Monrok 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 G5 



3. Argilloid trachyte, very heavy masses, probably several layers but divis- 



ional Unes not readily made out, dark-colored porphyritic crystals, much 



weathered on all surfaces . 230 



4. Dolerite, large plagioclase crystals, dark-gray color 40 



5. Augitic trachyte (?), several layers 70 



G. 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 separ- 

 able 200 



