THE NATURAL HISTORY OF IGNEOUS ROCKS. 27 



produce such disturbances. What is true of the Tertiary 

 eruptions seems to be equally true of those belonging to 

 earlier systems. Thus, in the Caledonian region, rocks of 

 the alkali group occur in the Norwegian and of the sub- 

 alkali group in the British province. The point has been 

 touched upon in an earlier communication to this Journal/ 

 and need not be pursued further. 



Although what has been said would suggest that petro- 

 graphical provinces may to some extent have been 

 defined as such from the beginning, their individuality has 

 become progressively more marked as the evolution of rock 

 types proceeded, such evolution being advanced to different 

 stages or following different lines in the several provinces 

 of one region. The history of any one province during its 

 period of life is also, from the petrographical point of view, 

 a record of progressive specialisation, resulting in increasing 

 diversity and peculiarity of rock types. In a typical 

 province, conceived in the sense that we have indicated, 

 the igneous rocks belonging to the earlier stages are often 

 characterised by considerable uniformity and simplicity. 

 Later a greater variety may be developed, sometimes run- 

 ning into highly specialised types, which do not readily find 

 a place in any classificatory scheme. This seems to be the 

 case especially in provinces belonging to the Atlantic facies, 

 or in other words among rocks rich in alkalies. Specialisa- 

 tion is accompanied by localisation, and there may arise 

 distinct districts having peculiarities of their own. Such a 

 general law is not to be accepted without proper qualifica- 

 tion ; in particular we must suppose that the progress of 

 evolution may be very unequal in different cases, and may 

 be arrested by extinction of activity when only little 

 advanced. 



For illustration we recur to the Tertiary rocks of Pacific 

 facies in America. Iddings has compared the lavas of the 

 Andes with those of the Great Basin, between the Rocky 

 Mountains and the Sierra Nevada (5). The former are 

 almost exclusively andesites and dacites ; the latter include 

 similar types but, in addition, basalts and rhyolites. 

 1 " Science Progress," vol. i., pp. 152-165, 1894. 



