THE NATURAL HISTORY OF IGNEOUS ROCKS. 33 



author has shown that the Triassic rocks of the Monzoni 

 district present a rather close parallel to the Norwegian 

 series, and he gives reasons for believing that the order of 

 increasing acidity is one of wide general application, the final 

 reversion to basic types being also of frequent occurrence 

 though less characteristic (11). In many districts we find 

 among true plutonic bodies of rock and their immediate off- 

 shoots evidence of basic eruptions having been succeeded 

 by others of more acid composition, and sometimes a series 

 of several terms showing progressively increasing acidity. 

 Where, however, there has been a distinct later stage 

 characterised by dyke intrusions, the order seems to have 

 been often, if not generally, reversed, and the more basic 

 dykes are seen to cut the more acid. These and kindred 

 phenomena lead to theoretical considerations which will find 

 their place more appropriately in a future communication. 

 In what has now been written we have confined ourselves 

 to remarks which are the direct outcome of observation and 

 comparison. 



Alfred Marker. 



