THE EVOLUTION OF IGNEOUS ROCKS. 155 



differences. This is indeed illustrated in the special districts 

 in question: the Etna lavas here discussed are very distinctly- 

 more basic than the older volcanic rocks connected with 

 the same centre. 



In such volcanic districts as have been carefully studied, 

 a definite order of succession in time has been observed for 

 the different types of lavas erupted, and this order, despite 

 exceptions, seems to be a significant one. In numerous 

 cases the law seems to be that the earliest products are of 

 intermediate chemical composition (andesites, etc.), while 

 the later ones become progressively more acid, or more 

 basic, or both alternately or simultaneously, i.e., increasingly 

 different from the first. Two recently recorded instances 

 of this apparent progressive differentiation may be added to 

 those already well known. 



In the Eureka district of Nevada, Hague 3 gives the 

 following as the order of the eruptions in Tertiary times : 

 i. hornblende-andesites, ii. hornblende-mica-andesites, iii. 

 dacites, iv. rhyolites, v. pyroxene-andesites, vi. basalts. 

 Here the series i., ii., v., vi. is one of increasing basicity, 

 while i., iii., iv. is one of increasing acidity. 



In Mexico, 4 again, the Tertiary eruptions began in the 

 Upper Miocene with what are described as andesitic porphy- 

 rites and propylitic andesites, and these were followed by 

 extensive outpourings of hornblende-andesite. In Pliocene 

 times there followed in order : hornblende-mica-andesites, 

 hornblende - hypersthene - andesites, hypersthene - andesites, 

 augite- andesites, and labradorites, the last graduating 

 into the numerous basaltic flows which have continued 

 through Quaternary times. Here the general order is one 

 of increasing basicity, but the acid series is also represented, 

 though with less completeness. Rhyolites occur, of which 

 the precise epoch is not known, but is later than that of the 

 hornblende-andesites. 



The initial magma cannot be assumed to have been in 

 every volcanic district one of intermediate composition. In 

 Iceland, for example, is found an enormous extravasation of 

 basic material, while the intermediate and acid lavas are 

 much more scantily represented. Still rhyolites occur in 



