138 
IDDINGS. 
completeness than can be found in older analyses. It is to be 
regretted that there do not exist more extensive sets of com¬ 
plete and accurate analyses of the rocks of special localities > 
since they are necessary for the proper comprehension of the 
nature of igneous rocks as a whole. 
A series of analyses representing the different varieties of 
rock at Vesuvius was chosen from those made by Houghton 
and Ricciardi. To these were added analyses of the rocks of 
Ischia by Fuchs, and a facies of the Vesuvian magma occur¬ 
ring as a bomb, and two rocks from Rocca Monfina. They 
are given in Table IV, and the molecular variations are 
shown in diagrams 4, 4 a, and 4 b. 
Another group of analyses shows the composition of the 
lavas of Etna, Table V, with two basic rocks from Pantel- 
leria, and one from the Island of Ferdinandia, and another 
group gives the composition of the more silicious rocks of 
Pantelleria, Table VI. The molecular variations of these 
two groups are expressed in diagrams 5 and 6, and by 5a, 
5b, 6a, 6b. The two groups of rocks are geographically close 
to one another and belong to the saime general region. For 
present purposes it is not necessary to consider the miner- 
alogical character assumed by these magmas. A list of the 
rocks from which the analyses have been made will be found 
in connection with the analyses. 
The chemical individuality of any group is best appre¬ 
ciated when it is compared with the chemical character of 
some other, for with so wide a range of variability it is not 
always evident where the characteristic quality is to be 
found, and yet it is sometimes observed upon comparing 
groups as closely situated as those already mentioned in the 
Yellowstone Park region, for we are passing from the Great 
Basin type into the Eastern zone. The characteristic differ¬ 
ence between these two groups lies in the alkalies. In that 
at Electric peak and Sepulchre mountain the ratio between 
the soda and potash molecules is from 3:1 to 2:1, their 
variation from the less silicious to the more silicious end of 
the series being more gradual and less irregular than that 
of the other elements. 
