162 THE JOURNAL OF GEOLOGY. 
main constituents (except SiO,) are plotted together, while in 
the two lower sections they are given in two parts to avoid con- 
fusion and bring out more clearly some of the features of interest. 
I have arranged them somewhat differently from Iddings’ pre- 
vious diagrams, grouping together the CaO, Na,O and K,O, 
since these elements occur most prominently together in the 
feldspars, FeO and MgO occurring together in the ferro-magne- 
sian silicates, and with these last two I have placed the Al,O,. 
It is seen from this that alumina occupies a very high position 
though with considerable variation. In the more basic rocks it 
is surpassed three times by the CaO, which in this part of the 
diagram varies inversely as the Al,O, does, while later the two 
lines run almost parallel, the CaO being considerably lower, and 
dropping a great deal as the SiO, increases. CaO and MgO 
run also approximately parallel, though the drop in MgO is less 
than in the other, and the inverse variation of Na,O to both is 
well seen, especially with CaO and Na,O in the middle section. 
K.O follows the Na,O line fairly closely, but its variations are 
always less than those of the other constituents, and it remains 
throughout in the lowest position, Na,O being at least twice and 
often more than three times as great. It may be noted that in 
only two cases do the molecules of Na,O and K,O sum up more 
than those of CaO. AI.O, and FeO also vary inversely at the 
very end, where there is a considerable drop in the FeO. MgO 
and FeO have quite similar lines, though at one point their varia- 
tion is inverse, the latter maintaining an almost constant lower 
position. MgO varies on the whole inversely to K,O and still 
more markedly so to Na,O. 
The relations of the segregations to their enclosing rocks is 
beautifully shown; the three segregations come also together 
near the basic end, while the three hosts come nearly together 
towards the acidic end; the richness of the segregations in lime 
and magnesia, their poverty in soda, and the variable character 
of the alumina and iron being well brought out. 
Attention has already been called to the fact that the A¢gina- 
Methana rocks show many points of resemblance to those of 
