464 
JOHNS : EXPERIMENTAL PETROLOGY. 
they have persisted in a path that has led them nowhere in 
particular, and that they must retrace their steps if they are to 
accomplish what should and must be their real work, namely, to 
elucidate the life-history of the Igneous rocks. 
It might not be entirely out of place to discuss briefly the 
things that we are really seeking to know something about. If 
possible it would be desirable to know the composition and 
constitution of the original magma which fed the intrusive and 
contemporaneous masses we are dealing with. Its temperature 
and pressure, and the variations in these that resulted after 
extrusion had taken place. The conditions that determine the 
growth of porphyritic crystals — those that determine both 
regional and local segregation have not been stated. The 
physico-chemical changes that take place in the consolidating 
mass as the various minerals separate out, and the particular 
conditions that govern these changes suggest a series of 
problems whose complexity and magnitude will only become 
apparent when their solution is attempted. 
Merely stating the problems does not constitute, at least, 
not directly, any useful contribution to their solution. The 
first thing required is the adoption of a tentative theory, based 
on the little we know already, as a working hypothesis. In the 
study of slags and alloys this has been found in the theory of 
reciprocal solution. The cooling fused rock mass ma}^ be con- 
sidered as a complex solution. Which is solute and which is 
solvent is not always clear. That solvent may become solute at 
a certain critical point in the thermal history of the cooling mass 
is sometimes very evident. Now reciprocal solution is a con- 
ception sufficiently elastic for the early days of research, even 
if it does not furnish the key that will unlock the casket that 
contains all the treasures we are seeking. But if the theory of 
reciprocal solution be adopted, then we must commence to 
determine the solubility curves of the various minerals in the 
constantly changing mother liquor of fused silicates from which 
they separate when the fused rock mass is consolidating. 
The brilliant work of Vogt, confirming Teall's suggestion 
that micropegmatite is the eutectic mixture of felspar and quartz. 
