EICnTHOFEN'S ORDER OF SUCCESSION OF ERUPTIONS. 63 



living observer has studied this problem more carefully nor included in his 

 observations and generalizations a wider field. His extensive knowledge, 

 his great acumen, and his ability to generalize brilliantly, though cautiously, 

 entitle his conclusions to the most earnest consideration. As the result of 

 his study of volcanic phenomena in many portions of the world, he believes 

 that the various kinds of eruptive rocks reveal a certain order of succes- 

 sion in their relative ages of eruption throughout Tertiary time. Arrang- 

 ing these rocks according to their physical properties and intimate constitu- 

 tion into five groups, or orders, he finds that they have been erupted in the 

 following sequence • 



1. Propylite. 



2. Andesite. 

 ;5. Trachyte. 



4. Rhyolite. 



5. Basalt. 



It will seldom happen that more than two or three of these kinds of 

 rock will be found in direct superposition, the series in any given locality 

 being always incomplete, and in very many cases a single kind will alone 

 be found. But wherever two or more are found superposed, the one having 

 the prior enumeration in the foregoing list will be the older. The only 

 exceptions would be where each order of rocks is represented by numerous 

 individual outbreaks, when the later extravasations of the older order may 

 occasionally be seen to intercalate with the older extravasations of the later 

 order. These considerations apply to what are termed "massive eruptions," 

 where deluges of lava have broken forth from fissures and overwhelmed 

 the adjoining regions with coulees far exceeding the ordinary emanations 

 of common volcanoes. They also apply to the history of those grander 

 vents which have maintained an activity lasting through a considerable 

 proportion of Tertiary time. But the smaller vents as a rule are of very 

 brief geological duration, and seldom disgorge more than one kind of lava. 

 In support of his generalizations he adduces his own extended observations 

 in Hungary, Germany, and the Sierra Nevada, arid those of many colabor- 

 ers in Armenia, Mexico, Central and South America. 



Those geologists who have made a special study of the volcanic rocks 



