70 GEOLOGY: C. SCHUCHERT 
flow other than that which may be exerted by the superimposed hydrostatic 
pressure. From such experience as we have gathered in this laboratory, 
hydrostatic pressure can have no other effect than to raise the melting tem- 
perature 10 or 20° per thousand atmospheres, that is, 1 or 2%, and this fac- 
tor must therefore be accounted comparatively insignificant in determin- 
ing solidification. It is conceivable that great hy4rostatic pressure might 
have the effect of preventing the escape of the volatile ingredients contained 
in a sub-aqueous lava flow and so facilitate crystallization. It is our experi- 
ence that a very small quantity of such ingredients has enormously greater 
influence in determining crystallization than a very large hydrostatic pres- 
sure alone. It might therefore follow that the pressure operating in this 
indirect manner might serve to keep volatile ingredients 'on the job,' so to 
speak, which would otherwise escape and so promote crystalUzation in a 
mixture which would otherwise tend to cool in vitreous form. Beyond this 
possibility I can conceive of no basis in present experience for the assertion 
which Termier has made. In general, silicate mixtures which crystallize 
with difficulty will form glass if cooled quickly whether under pressure or 
not — the pressure apparently being the least important factor in the situa- 
tion. Similar mixtures which crystalHze readily can with great difficulty 
be cooled quickly enough to prevent crystallization, and here again the fac- 
tor of pressure is relatively insignificant. 
You may recall a paper by Johnston two or three years ago in which he 
showed plainly and unmistakably that in general small changes of tempera- 
ture or concentration would have greater effect in determining the resulting 
soUd form than a thousand atmospheres of pressure. This conclusion is in 
a sense obvious, for if a thousand atmospheres will produce no more than 
10 or 20° effect on the melting temperature, then obviously 10 or 20° tem- 
perature change in this temperature region will be its equivalent. In the 
same sense a 1 or 2% admixture of one of the volatile ingredients will produce 
several tens of degrees lowering of the melting point of the solution in this 
temperature region. These considerations are perfectly general and apply 
without reservation to the condition of things which Termier is discussing. 
I am therefore disinclined to give any weight to the evidence which he ad- 
duces in proof of the contention that vitreous basalts could not have formed 
at depth as well as anywhere. 
This paper has also been read by Prof. L. V. Pirsson, and he makes 
the following comments in regard to the formation of tachylytes: 
Whether a magma will soHdify in a vitreous or a crystalhne condition ap- 
pears to be much more due to temperature than to pressure. The latter, 
in the quantities which we have to deal with in the superficial crust of the 
earth, seems relatively neghgible compared with very moderate changes of 
temperature. If the change of temperature of a basaltic magma on attain- 
ing a sub-aerial surface is sufficient to cause it to solidify as a glass, or tachylyte, 
