366 Tlic American Geologist. Juno, is59 
III. Inclusions. These are so frequent and they are so 
much corroded that they testify to the great importance of as- 
similated masses. The student may examine the work of La- 
croix: "Enclaves des roches volcaniques." 
W . The contacting surfaces of dikes. In the case of cer- 
tain dikes, not very numerous, however, the contacts present 
evidences of fusion, and the dike contains inclusions derived 
from the wall. Lewinson-Lessing has observed such at Cauca- 
sus, in the granites of Darial, and in dikes of diabase. Marker, 
in "TheCarrock Fell," cites such inclusions in the gabbrosand 
granophyres. 
\\ Minerals accompanying contacts. These have been 
fully set forth by Lacroix (Enclaves des Roches, etc.) and by 
others. 
This theory of assimilation is called "osmotic," by John- 
ston-Levis because it appeals to osmose to explain the inter- 
change of materials that goes on between the magma and the 
inclusions. The author does not hold that view. When resorp- 
tion takes place on a small scale, there is a production of con- 
tact minerals. When the magma resorbs great mineral masses 
there is dififerentiation ; for instance, the resorption of lime- 
stones or dolomytes by an acid magma transforms the latter 
into a neutral magma, but, after a certain limit there occurs a 
separation into two magmas, an acid with alkalies and a basic 
with alkaline earths. The principal phenomenon of differentia- 
tion is the division into two magmas: — feldspathic and ferro- 
magnesian, but this phenomenon is the product of liquation 
and not by osmose. Liqtiation can be instigated by different 
causes: by inclusions, by variation of pressure, etc. 
It might be thought that the resorption of great rock 
masses, requiring heat, could not take place on a grand scale. 
But in the greater number of cases the magma becomes more 
fusible after resorption; for instance, the magnesian magma 
becomes more fusible after the resorption of lime, and conse- 
cjuently capable of new assimilations. And, further the tem- 
perature of fusion of a mixed body is less than temperature of 
the constituent bodies. For difTferentiation it is also necessary 
to consider the role which the different oxides can play. Vogt 
says that the magma becomes more fusible when it becomes 
rich in FeO, MgO or MnO. On the contrary K^O and Al-O.-' 
