384 The American Geologist. Dpcember, isuo 
that, although certain of the principal advantages of the lat- 
ter are lost in the modified diagrams, this method has certain 
advantages peculiar to itself. (See plate XIV.) Brogger pre- 
fers to use the relative number of molecules of each oxide 
instead of the relative weights, as used l^y Michel Levy. The 
value of SiO^ is carried on the horizontal axis, half in each 
direction from the center. The CaO is represented on the 
positive and the Al^O^^ on the negative vertical axis. Two oth- 
er axes are added, making angles of 30° with the horizontal, 
and. on these the Na-O is represented to the left below, the 
MgO to the right above, and the FeO to the left above. To 
the last is added the Fe-^^Os. 
In the plate XI\', figure 3 represents the average of the 
olivine gabbro (1136) d^'abase (954) and olivine diabase (1843). 
The figure indicates the practical absence of potash, low sodi- 
um oxide, and rather high calcium oxide, about equaled by 
the magnesia, and the sum of the oxides of iron. The high 
per cent of calcium oxide and of alumina, and very low per 
cent of magnesia, iron and potash of the plagioclasyte (Fig. 2) 
appear very clearly. The extreme abundance of the iron is 
the feature of the silicoferrolyte (Fig. 4). The high per cent 
of magnesia and iron oxides is the distinguishing point in the 
troctolyte (Fig. 6). The alumina of the cordierite noryte (Fig. 
5) is high in proportion to the calcium oxide, but the excess 
does not appear. A careful examination shows the increase 
of potassium oxide in the orthoclase gabbro (1797) (Fig. 7). 
Similarly a careful examination is necessary to detect the in- 
crease of silica in the quartz gabbro (854G) (Fig. 3.) 
If now the point of view be changed, and the series be con- 
sidered with Becke from the basal standpoint of the relation 
K : Na : Ca, it will be found that the chief features of the 
various types are largely lost to view. Indeed, this relation 
between the alkaline-earth metals is the only one conspicuous- 
ly kept in view in the diagrams of Becke. (See plate XVI.) 
Becke considers two rectangular axes on which he lays oflf Na 
= + I on the X axis, and K = + i on thej/ axis., Then, for 
any given analysis, the co-ordinates n and k locate a point 
which is considered characteristic for the analysis. These co- 
ordinates are obtained from the relations : 
Na - Ca _ K - Ca 
'' " Ca+Na + K ' ' ~ Ca-fNa + K ' 
