MONROE AMPHITHEATER— PROPYLITES. 229 



tions presented. The most conspicuous rock of the oldest series is a ridge 

 of hornblendic propylite extending across the opening of the amphithea- 

 ter. The stream which drains the amphitheater has cut a cleft 20 or 30 

 feet wide and more than 500 feet deep through this barrier (Heliotype I), 

 and the gorge has received the name of Grate of Monroe. The length of 

 this chasm between propylitic walls is about half a mile. Following it 

 downstream the massive propylite gives place suddenly to beds of con- 

 glomerate and clay, baked and altered by heat, which abut in the natural 

 section against the propylite. They are probably younger than the vol- 

 canic rock and may have been derived from its waste. At the upper end 

 of the gorge the propylitic mass ends suddenly — a lateral ravine parallel 

 to its precipitous face hiding its mode of exit. On the other side of the 

 ravine is a mass of andesite succeeded by trachyte, both apparently younger 

 than the propylite. The propylitic mass may have been erupted at as early 

 a period as Middle or Late Eocene, for the stratified beds which abut against 

 its western flank have evidently been water-laid, and there is no evidence 

 of the existence of any considerable body of water in this locality later 

 than the epoch referred to. Moreover, beds of similar nature, sometimes 

 altered, sometimes not, are found around the eruptive centers in many 

 localities, and have been derived from the destruction of some unknown 

 volcanic rocks. Fragments of similar altered rocks are brought down by 

 the stream from some of the forks above, showing that on both sides of the 

 propylitic mass these peculiar sediments were deposited. Very partial 

 exposures of propylitic rock are also found elsewhere in the deepest part 

 of the ramifying gorges, cut by the many streams that unite in the creek 

 which cuts the cleft in the larger barrier of the amphitheater. 



These propylitic rocks are interesting, inasmuch as they furnish another 

 instance of that priority in time among Tertiary eruptions which Richt- 

 hofen has claimed for them. Here they are not only older than all other 

 eruptives, but they appear to speak of an epoch in which they alone were 

 erupted, and that epoch probably goes as far back as the Middle Eocene. 

 They certainly do no appear among the later or the middle eruptions. A 

 period of rest from volcanic disturbance succeeded their extravasation, and 

 during that quiescent pei'iod they were much ravaged by erosion. Patches 



