380 The American Geologist. December, isdo 
The scale adopted by M. Michel Levy is two millimeters 
for one per cent; it is too small to properly represent the 
amount of potassium oxide found in many basic rocks, but 
as a larger scale would in some cases produce a diagram too 
large, the same scale is employed in the diagrams. (See plate 
XIII). 
An examination of the diagrams constructed according to 
the rules just established and based on the values furnished by 
the analyses will show at once that the olivine gabbro (Plate 
XIIL, Fig. 3) and olivine diabase (Fig. 6} can be considered 
together, being nearly identical in all respects. They are rather 
basic rocks as shown by the slight development of the felds- 
pathic triangle below the axis x , and the dominant feldspar is 
labradcrite, as shown by the ratio : CaO : Na^O. They are 
not saturated with alumina but the calcium excess (6"') is small; 
the magnesia is approximately equal to the calcium of the 
feldspars. Both types are rich in feldspar, as shown by the 
relative area of the two triangles.* The ratio K:;0 : Na^O 
varies only slightly from i : 5 to i 14. The only differences 
to be detected are : First, the feldspar elements are a trifle 
more abundant in the olivine diabase ; second, the magnesia 
and iron oxides are a trifle less, and the calcium .excess a trifle 
greater in the ophitic type. The diabase (Fig. 4) belongs to 
the same group, and differs only in showing less magnesia 
and more iron, corresponding to a decrease in the amount of 
the olivine. The ratio K^O : NasO remains i : 5, and the 
feldspar is labradoric. These types mineralogically certainly 
belong to the gabbro family and are derived from the magma 
called "diorito-diabasic" by Michel Levy (except the diabase 
(954), which is from the"granito-esterellic" magma), that is, 
a magma in which the calcium oxide of the feldspars is in 
large amount, but is approximately equaled by the magnesia, 
while the potassium oxixle is nearly wanting. On the con- 
trary they present the inequality, Fe i- Mg > Ca + Na + K, 
which is characteristic of the peridotyte magma (tt), of Ros- 
enbusch, though at the same time they present the charac- 
ter of his gabbro magma (V), namely, Ca > Na + K, Si > 
*This is more evident, when it is noted that the maximum area of 
the feldspar triangle (cf. plate XIII, Fig. 7) does not exceed one- 
fourth to one-sixth of the maximum area of the ferromagnesian tri- 
angle (cf. Fig. 10.) 
