94 E. H. L. SCHWARZ 
comparatively narrow supply dykes, the channels of supply and 
escape of waste siliceous material are so small that the diffusion of 
the substances in the magma would have been hindered and as a 
result these dykes consolidated with rocks of the average composi- 
tion. 
THE DIFFUSION DUMBBELL 
Taking the whole evidence which the granite bosses, the 
diorite dykes and the dolerite laccolites afford us in Cape Colony, 
one is led to conceive of a system of igneous injections of a dumbbell 
shape. Below we have the Malmesbury clayslates, above the 
Karroo sediments, in between the various siliceous sediments of the 
Cape system, or sometimes in the north of the Pal-Afric group 
Kheis quartzites and Pretoria iron-bearing quartzites. Below, the 
magma eats out great holes and fills them with granite; above, the 
same magma eats out the holes and fills them with basic rock. In 
between are thin dykes of communication usually dolerite but 
uuder certain conditions diorite. The slates above and below 
became absorbed and the material from both was added to the 
general stock of magma. The average magma remained fluid in 
this dumbbell system for some time, till for some reason the acid 
part concentrated in the lower part and the basic in the upper part, 
the diffusion taking place through the narrow part of the dumbbell 
and sometimes the average magma became caught in this part and 
became consolidated as diorite. 
The facts are plain enough and it is perhaps as well to content 
oneself with them at present, but there is an explanation which, if 
only speculative, may be worth mentioning in order to show that 
this differentiation of an average magma is not wholly unexpected. 
THE IONIC SEPARATION OF SUBSTANCES IN A ROCK-MAGMA 
When an iron-bearing rock weathers at the surface of the earth, 
the iron does not travel outward to the sea along with the soda, 
lime, and potash but seeks the center of the earth.7 The fact is 
easily recognizable in the replacement of limestone by iron ores, 
*E. H. L. Schwarz, Causal Geology (1910), 73; ‘‘Selective Absorption of Sub- 
stances in the Earth’s Crust,” Journal Assoc. for Advancement of Science, S. Africa 
(Cape Town, 1012), 181. 
