64 
DUTTON. 
uplifts it in no manner affects the law of isostasy. What 
the real nature of the uplifting force may be is, to my mind, 
an entire mystery; but I think we may discern at least one 
of its attributes, and that is a gradual expansion, or a dimi¬ 
nution of the density, of‘the subterranean magmas. If the 
isostatic force is operative at all, this expansion is a rigorous 
consequence; for whenever a rise of the land has taken 
place one of two things has happened: the region affected 
has either gained an accession of mass or a mere increase of 
volume without increase of mass. We know of no cause 
which could either add to the mass or diminish the density, 
yet one of the two must surely have happened. But the 
difference of the two alternatives in respect to consequences 
is immense. If the increase of volume of an elevated area 
be due to an accession of matter, the plateau must be hoisted 
against its own rigidity and also against the statical weight 
of its entire mass lying above the isostatic level. But if the 
increase of volume be due to a decrease of density there is 
no resistance to be overcome in order to raise the surface. 
Hence I infer that the cause which elevates the land involves 
an expansion of the underlying magmas, and the cause 
which depresses it is a shrinkage of the magmas. The na¬ 
ture of the process is, at present, a complete mystery. 
Note.— The foregoing paper was written hastily to occupy a vacant half 
hour of a meeting of the Philosophical Society without the thought of pub¬ 
lication. I have yielded however to the kind solicitation of friends to con¬ 
sent to its publication. It contains a rough outline of some thoughts which 
have worked in my mind for the last fifteen years and which, from time to 
time, I have discussed at length in unpublished manuscripts and in familiar 
conversation with my esteemed colleagues. 
