166 THE JOURNAL OF GEOLOGY. 
Such differences in the fundamental chemical relations of the 
rocks of two widely separate regions are of very great interest 
especially, as Iddings has already pointed out,’ as showing that the 
great bodies of magma beneath various volcanic centers or fracture 
lines differ radically from each other in chemical composition, 
and that the ideas of the consanguinity of rocks and the differen- 
tiation of magmas are true ones and based on deeper and more 
real distinctions than differences in mineralogical composition or — 
the habit of the component minerals, which seems to have been 
the main idea in the minds of the originators of the term ‘ petro- 
graphical provinces.” ? 
In order to test this view further and set the reality of the 
term ‘‘petrographical province”’ more clearly and convincingly 
before the reader, I prepared Diagram 2, which represents the 
molecular variation of the eruptive rocks of Santorini, as given 
by the analyses shown in Table II. This diagram can only be 
regarded as an attempt at expressing the relations between the 
Santorini rocks, since Fouqué’s analyses are most unsatisfactory 
(as is evident from an examination of them) and the probably more 
correct analyses quoted by Roth chiefly embrace the more acid 
rocks of Thera and the later eruptions. More analyses of the 
various lavas of this group are much to be desired and a diagram 
prepared from them would be much more conclusive than the 
present one. 
Notwithstanding the unsatisfactory character of the analytical 
material the resulting diagram surpassed my expectations in 
regard to the similarity between the two districts, the two show- 
ing the same character in almost every respect, even down to 
details. The similarity will be evident at the first glance and the 
remarks on Diagram 1 will answer for Diagram 2, but to make it 
more emphatic I may call attention to a few points. 
Na,O and K,O are in both beautifully parallel and with much 
the same habit, though in the Santorini rocks they drop a little 
at the acidic end, which, it must be remembered, is slightly higher 
t Orig. Ign. Rocks, p. 135. 
?VOGELSANG, Z. d. d. Geol. Ges. 1872, p. 507. JUDD, Quart. Jour., Geol. Soc. 
1886, p. 54. 
