Interior Condition of the Earth. — Le Conte. 43 
probably in outline as follows : In the originally incandes- 
cent liquid condition of the earth, as long as the liquidity was 
perfect, convective currents would tend to keep the whole 
mass at nearly equal temperature throughout. The cooling 
mass therefore commenced to solidify at the centre, because 
the pressure was greatest there. The nucleus thus formed, 
continued to grow in size until it nearly reached the surface. 
But when the surface liquid by cooling had become viscous, 
so that convective currents were impeded a superficial solid 
crust would be formed by radiation. The sub-crust liquid 
thus formed was now encroached upon on one side by the 
solid crust and on the other by the solid nucleus until at pres- 
ent it is comparatively thin or perhaps abolished entirely in 
many places and the crust united with the nucleus. This is 
probably the condition of things now ; for there are some geo- 
logical facts, especially the existence of those greatest features 
of the earth surface, constituting continental masses and 
oceanic bottoms, which seem inconsistent with the existence of 
a universal sub-crust liquid obeying the laws of hydrostatic 
level. 
The original liquid magma, undoubtedly increased in densi- 
ty to the center ; and that, not only by increasing pressure but 
by difference of materials and especially by superficial oxida- 
tion. Therefore the solid crust may be regarded as Hoating on 
the sub-crust liquid even though (as is almost certain) rocks 
become denser by solidification. The instability of such a 
floating crust under the weighting by sedimentation and light- 
ening by erosion is sufficiently obvious. But these effects 
may be increased in the manner suggested by Lloyd Morgan.' 
Sedimentation increases the weight and therefore produces 
subsidence of the floating crust ; but increased weight means 
increased pressure, and increased pressure produces solidifica- 
tion of the upper parts of the sub-crust liquid, and solidifica- 
tion means increased density and therefore contraction, and 
this again produces further settling down of the crust. Con- 
trarily, erosion by lightening the floating crust produces 
elevation, but lightening the pressure produces fusion of 
the lower portion of the crust, and fusion produces expansion, 
and this again in its turn increases the elevation. In some 
such way we may account for the pari passu subsidence under 
'Geol. Mag., vol. 5, p. 291. 1888. 
