THE NATURAL HISTORY OF IGNEOUS ROCKS. 25 



folding, these provinces often differ from one another in 

 respect of their petrographical characteristics. This is well 

 seen in the ligrht of the two-fold division of ioneous rocks 

 just enunciated, and the evidence is naturally clearest in 

 the case of the Tertiary and post-Tertiary eruptions. 

 Iddings has already drawn attention to the remarkable 

 geographical distribution in the New World of the two 

 petrographical series, which he names respectively the 

 alkali and the sub-alkali group (4). Along, and to the west 

 of, the great continental water-shed of the two Americas the 

 vast areas of igneous rocks belong to the sub-alkali group. 

 On the eastern slopes of the Rocky Mountains, however, in 

 Montana, Wyoming, Colorado, and Te.xas, rocks rich in 

 alkalies appear, and farther east numerous areas char- 

 acterised by rocks of the alkali group occur in Canada, the 

 New England States, New Jersey, and Arkansas, and 

 in Brazil, Argentina, and Paraguay. Some of these rocks, 

 it is true, are of pre-Tertiary age, and we probably have to 

 deal with a case of the superposition of newer upon older 

 provinces already alluded to. Apart from this question, 

 however, we are justified in stating summarily that, of the 

 latest great system of igneous rocks in America, the alkali 

 and the sub-alkali groups are developed on the Atlantic and 

 the Pacific slopes respectively. 



In Europe the distribution of the two groups is equally 

 clear, though not to be summarised in such simple terms. 

 Judd long ago drew attention to the strong contrast between 

 the Tertiary igneous rocks of Bohemia and those of 

 Hungary, divided only by the Carpathian chain. The 

 one set belongs to the alkali, the other to the sub-alkali 

 group. Rocks of the former group are developed along the 

 northern border of the Alpine zone of folding, from Bohemia 

 to France, and in the Tyrrhenian province, while the rocks 

 of the Hungarian, Servian, and ^gean provinces are 

 relatively poor in alkalies. The Brito-Icelandic province, 

 partly in consequence of the very sparing development of 

 intermediate rock-types, is less sharply characterised. The 

 rocks as a whole seem to belong decidedly to the sub- 

 alkali group, though they are not without anomalies. 



