THE EVOLUTION OF IGNEOUS ROCKS. 153 



indicate that very different magmas may be derived from 

 the same ultimate source. Taking first volcanic rocks, it is 

 noteworthy that though the lavas erupted at different times 

 within one volcanic district may differ widely in chemical 

 composition, ranging sometimes between very acid and very 

 basic types, yet there are often striking chemical peculiarities 

 which run through the whole group of rocks belonging to 

 one volcanic centre, differentiating them from those of other 

 centres, and only to be explained by some kind of con- 

 sanguinity among the associated lava-flows. This is well 

 brought out in a paper by Iddings, 1 in which he reviews 

 the general subject of the origin of igneous rocks, and draws 

 illustrations especially from those studied by him in the 

 Yellowstone Park region. These are discussed under three 

 heads. The volcanic rocks of Electric Peak and Sepulchre 

 Mountain range in silica-percentage from 56 to 69, but still 

 have distinctive characters in common, such as the constant 

 predominance of soda over potash, the molecular ratio of 

 the two varying from 3:1 to 2 : 1. The rocks of the old 

 volcano of the Crandall basin show a greater range of 

 composition, the silica-percentage being 52 to 71 J- in 

 different examples, but they have peculiarities which link 

 them together. The alkalies are here more plentiful, and 

 the predominance of soda over potash less marked, the 

 molecular ratio being between 2 : 1 and 1:1. In the 

 peculiar dykes and flows of the Absaroka range, with a 

 silica-percentage ranging from 47 to 69J, the alkalies are 

 present in still greater force, and are about equally 

 represented. Iddings compares these three groups of 

 rocks with those of the well-known Italian volcanoes, also 

 rich in alkalies, and finds that each district has its own 

 chemical characteristics. In the case of Vesuvius the 

 molecular ratio of soda to potash varies from 7 : 4 to 1 14. 

 In the Etna lavas the ratio is much higher, rising even to 

 8:1, while lime and magnesia are very abundant. In the 

 peculiar rocks of Pantellaria the ratio is from 5:1 to 2 : 1. 

 Here iron-oxides are richly present, and increasingly so in 

 the more acid rocks. Iddings shows these various relations 

 graphically, taking the molecular numbers of silica as 



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