142 
IDDINGS. 
explains the association of various kinds of volcanic rocks, 
such as those in the regions selected for discussion, and also 
shows the error of attempting to correlate all kinds of rocks 
on a simple basis of silica percentages. Thus the natural 
series of volcanic rocks in the Yellowstone Park, arranged 
according to the approximate percentages of silica, is: 
Silica 
percentages. 
48-53 Basalt. 
55 Hypersthene-basalt. 
58 Pyroxene-andesite. 
61 Hornblende-pyroxene-andesite. 
65 Plornblende-mica-andesite. 
68 Dacite. 
70-75 Rhyolite. 
The series of exceptional dike and flow rocks of the Ab- 
saroka range is: 
47-53 Leucite-orthoclase-basalt (phono litic basalt). 
53-56 Basaltic-trachyte. 
57-61 Trachyte. 
69 Rhyolitic-trachyte. 
The series of rocks from Vesuvius and Ischia is: 
46-55 Leucitophyre. 
55-62 Trachyte. 
69-71 Rhyolite (Ponza). 
The series from Pantelleria is: 
49 Basalt (doleritic). 
50-56 Phonolite. 
61 Augite-andesite (with cossyrite). 
67-70 Pantellerite. 
70 Rhyolite. 
In these series even those rocks to which have been given 
the same names differ from one another very considerably 
