Joty— The Order of Formation of Silicates in Igneous Rocks. 303 
base-line, we should find that the quartz has retained some fluidity 
below the temperature at which the augite is solid. 
In so far as this diagram is hypothetical, it derives support 
from the observed more rapid liquefaction of the more basic silicates 
at high temperatures. The steeper rise of the fluidity curve for 
augite at high temperatures (that is the part prolonged above f, /) 
is, in fact, matter of observation. This fact is very probably also 
shown in the volume-change at these temperatures.' 
The true melting point, from the present point of view, is 
only to be obtained by observations on the behaviour of bodies 
under distorting forces. ‘These observations I am now making, 
but they will be tedious, and delayed by other work. Unfor- 
tunately, while the observations of viscous yield will undoubtedly 
afford valuable data, the possibility of obtaining estimations of 
the molecular forces concerned in bringing about crystallization, 
and so completing the test of the theory, is not so apparent, unless 
we may consider that the resistance to distortional force offered 
by the crystallized substance involves this quantity. But even 
should direct observation fail to completely elucidate the question 
at issue the knowledge of the viscous curves must enter any 
complete theory of the order of solidification of the silicates. It 
appears, too, that the experimental means affording these data 
may be extended to reveal how far hysteresis attending the solidi- 
fying points may exist, and, if so, whether this is related to the 
percentage of silica present in the silicate: a line of inquiry at 
once suggested by the views expressed in this note. 
* “On the Volume-Change of Rocks and Minerals attending Fusion.” By J. Joly.. 
Trans. Royal Dublin Society, New Series, vol. vi., p. 283. 
SCIENT. PROC. R.D.S., VOL. IX., PART III. P Fo 
