A TYPE OF IGNEOUS DIFFERENTIATION 641 
idea of convection with a rhythm of crystallization would lead one 
to expect exactly what is here found.t. The first crystals to form, 
if not altogether too light, would be the first to drag along the 
bottom. The heavy minerals would take their turn, and the mag- 
netite being, at least partly, later in time of crystallization would 
remain liquid until the lower parts of the chamber were filled with 
layers of rock. Thus both the uniform feldspar and the curve of 
gravity may be taken as signs of convection. 
20 
10 
Number of analyses. 
Per cent Silica 
Fic. 9.—Silica content of specimens of the Duluth gabbro formation 
Double differentiation —A second feature of the rocks of the 
lopolith, which has a bearing on the process of differentiation, is the 
apparent break in the series. This is evident in plotting curves of 
variation in chemical or mineralogic constituents, and even more 
strikingly in a mathematical arrangement of the quantitative classi- 
fication. Nothing in the outline of crystallization differentiation 
leads one to expect any sudden changes in rock types or any omis- 
sions in the series of intermediate rocks. A mass of rock with 
‘Frank F. Grout, “Two-Phase Convection in Igneous Magmas,” Jour. Geol., 
XXVI (1918), 481. 
