EXDLicH.] VOLCANICS. 209 



into consideration primarily the stratigrapliical relations as above given, 

 the conclusion will present itself that this point, or very near this point, 

 must have been the main region of outflow; that here is the center from 

 which the greater portion of the volcanic material spread over the sur- 

 rounding country. In support of this view are the additional facts 

 that, at the locality in question, rhyolite and basalt, two of the youngest 

 volcanic rocks of ttiat region, were found in a caiiq;3 in considerable 

 masses, while at all other points they occupied either the summits of 

 peaks or of extensive plateaus. It seems strange, however, that these 

 same rocks found here, should occur elsewhere at such considerable 

 altitudes. Taking the evidence derived from strati graphical conditions 

 it would seem that, after the flow of volcanic matter had subsided, a 

 depression of the place from where it was issued took place. 



Another point of great interest and importance is that touching the 

 conditions of the country at the time of the outflow. It was observed 

 throughout, that in the eastern portion of the district the lower num- 

 bers of the trachyte prevailed, to the absolute exclusion of the highest 

 ones, while in the western localities this order of things was reversed. 

 Inasmuch as the four adopted subdivisions are found occurring con- 

 formably, one upon the other, the view that they must have originated 

 from one point, receives a decided support. Should this be the case, 

 then, the facts illustrated by the horizontal distribution of the volcanic 

 material can best be explained by the assumption that during the 

 period of the earlier flows the western country was too high to be in- 

 vaded, while during the later flows the order of things was reversed, 

 and the eastern portion received none of the younger material. At the 

 time of the succession of flows, the region east of station 10 must have 

 had a by far greater elevation than at present, a fact which is demon- 

 strated by the occurrence of the youngest volcanic rocks at higher alti- 

 tudes. It would be extremely difficult to attempt a thorough analysis 

 of the dynamics involved in these grand demonstrations of volcanic 

 activity, and particularly so after a survey that has been so limited in 

 time as ours. 



14 H 



