028 Correspondence—Mr. O. Fisher. 
CORRESPONDENCE. 
REPLY TO PROFESSOR PERRY’S COMMENTARY ON 
PROFESSOR MILNE ON VOLCANOS, 
Srr,—Professor Perry has in effect charged me with committing 
what he calls the “common error of many geologists, who know a 
little mathematics, of imagining that they can create a mathematical 
theory fora phenomenon.” It would be well if this sort of attempt 
were a little more common, and that geologists would apply a little 
more frequently the test of “how much” to their theories. Should 
errors be made they could be detected and exposed. 
But in the present case this rebuke is scarcely merited ; for, if your 
readers will look at Prof. Milne’s paper, towards the foot of p. 169, 
they will see that the effect of pressure in retarding fusion, referred 
to by his apologist, is not even alluded to by the writer: but solely 
the effect of the coldness of the bottom water of the ocean in lower- 
ing the position of “any given isotherm.” 
For determining that point I believe that my figures give the 
simple consequence of the acknowledged law of mean increase of 
temperature ; and I can scarcely be accused of having attempted to 
‘create a theory ” thereby. Perhaps however I lacked caution when 
I used without qualification the expression “melting temperature.” 
So I will explain what I meant by it. 
We must, I suppose, accept as a demonstrated fact, that the earth 
is ‘as a whole” extremely rigid. We believe that its interior is 
extremely hot. From these two propositions taken together it 
follows that the pressure to which the internal parts are subject 
induces solidity in matter which would otherwise be fluid through 
heat. This conclusion may be accepted without appealing to ex- 
periments, the results of which seem doubtful, respecting the floating 
or sinking of solid in melted rock. We know further that the 
superficial layer (which may be called the crust) is solid from cold. 
But we do not know whether there is a continuous and constantly 
liquid layer between the crust and that nucleus which is maintained 
solid by pressure, in spite of its high temperature. For my own 
part I suspect that there is. 
We may feel sure however that at a certain depth the rocks are 
at such a temperature that, if not already fluid, they would become 
so if relieved of the superincumbent pressure. This temperature 
would be the same as that at which they melt at the surface, and is 
what I meant by “the melting temperature.” Used thus, the term is 
merely a name for a particularisotherm. The temperature, at which 
the rocks might become fluid in spite of the pressure upon them, is 
more correctly termed “the melting temperature for the pressure.” 
Prof. Milne’s words on which I commented, do not make any 
reference to this condition. 
Perhaps it may be asked—what has the melting temperature at 
the surface, where there is no superincumbent pressure, to do with 
the theory of volcanos ? 
If we suppose the crust to be crumpled by lateral pressure, the 
vertical pressure beneath the anticlinals must be thereby relieved. 
