386 The Ameticati Geologist. December, 1900 
highest siUca of the series ; aluminum is relatively constant 
near twenty per cent, with a maximum of thirty and a mini- 
mum of ten per cent. ]Magnesia is the lowest element, only 
twice exceeding ten per cent, and then only attaining twelve 
and fifteen per cent. 
Iddings was the first to propose a graphic representation 
of rock composition, and his method is remarkably well adapt- 
ed for the purpose for which it was devised, namely, to illus- 
trate the consanguinity in certain series. His primary ar- 
rangement of the rocks is based on their relative acidity as 
given by the total per cent of SiO^. The silica of each type is 
laid ofT as an abscissa, and from the points thus determined 
the various oxides are measured as ordinates ; the alkalies and 
alumina in one diagram and the calcium, iron and magnesium 
oxides on another. The method thus has the marked advan- 
tage over the Becke diagrams of being incomparably more 
simple ; neither one, however, is well adapted to illustrate a 
single rock. 
In the plate (see plate XV), the original simplicity of the 
diagrams has perhaps been somewhat masked by the addi- 
tion of several features.* Thus, {o the diagram of the al- 
kalies and alumina have been added the feldspathisable cal- 
cium oxide and the silica of the feldspars, while in two cases 
the alumina has been divided to indicate the presence of an 
excess. To the diagram of the ferromagnesian elements have 
been added the silica of those elements, and the excess of cal- 
cium oxide, occasionally replaced by the excess of alumina 
which is laid oflf in the negative direction. 
Examining the upper diagram, the most striking feature 
is the quite constant proportion between k, 71 and c in the 
plagioclasyte, the olivine diabase, olivine gabbro, the diabase, 
the troctclyte. and the silicoferrolyte, and the variations in 
the three other rocks, (dotted lines). In four of these six 
types the proportion is almost exactly 1:5:15. In the oli- 
vine diabase this becomes i : 4 : 9; and in the plagioclasyte 
it is I : 9 : 25. Omitting the potassium the ratio is nearly 
I : 3 in all the types except the olivine diabase, where it is 
I : 2.3. ^ 
*Most of these modifications arc as used by Michel Levy. op. cit. 
p. 363, and plate XVI. 
