12 The American Geologist. January, loos 
■class, i. e., the eutectics. These, in my opinion, will lead to 
a rational classification of igneous rocks, themselves mix- 
tures and incapable of logical description except in terms 
of standard mixtures, the eutectics. 
It appears to me highly probable, for many reasons, that 
the magmas of the granular rocks are not liquids but stiflf 
emulsions, comparable with modeling clay, the solid constit- 
uents (perhaps free oxides) being merely moistened with 
magmatic liquids. Such masses behave mechanically like 
soft solids ; tiiey display some rigidity and in them diffusion is 
reduced to a vanishing quantity. They may be ruptured and 
the (aplitic or pegmatitic) liquid portion may then seep into 
the cracks. Such magma might be forced into minute fis- 
sures, as is the case when clay is molded to terra cotta 
articles and yet it would support permanently, on its upper 
surface, rocks of superior density. Only in such a magma 
can I comprehend the simultaneous growth of crystals of 
various minerals ; for in a liquid not exactly eutectic, the 
formation of crystals must follow a definite order. Again, 
if banded gneisses and gabbros had been fluid, the bands 
would show evidences of diffusion which as a rule are absent 
or barely traceable in these rocks. 
The relations between consanguineous massive rocks 
have occupied a large part of the attention of geologists for 
many years. At one time it was supposed that homogene- 
ous liquid magmas might split up into two or more homo- 
geneous magmas by processes of molecular flow due to dif- 
ferences of osmotic pressure. This process was called the 
differentiation of magmas. It has been shown, however, 
that these processes are so much slower even than heat dif- 
fusion, that they cannot be efficient beyond distances of a 
few centimeters. For this reason, Mr. Teall,* who first 
suggested the application of the Soret process to account 
for differentiation, professor Broggert and others, have 
abandoned the hypothesis of differentiation on a considerable 
scale by molecular flow. Nevertheless, observations on lac- 
coliths and other occurrences leave no doubt that a single 
magma may solidify to different though consanguineous 
* Froc. Geol. Soc. London, vol. 57, 1901, p.lxxxv. 
tEruptivgesteine der Kristianiagebi etes, part III, p 339. 
