Rise of Lava Caused by Gravity. 
495 
denudation, or the uprise of an arch of rigid rock supported in 
part by its limbs, or to deep-seated fractures, or to all of these 
combined, the rock below changes to a less rigid condition 
than normal for a given depth, or possibly into a liquid condi¬ 
tion, this modification of form is due to gravity, for it has 
already been seen that all of these processes in the final analysis 
are gravitative. In this change it is entirely possible, perhaps 
probable, that the heat of dynamic action, as suggested by 
Mallet, may be an important factor, although this factor does 
not have the dominating value which he attributed to it. 
Rise of Lava Caused by Gravity. — The plastic or liquefied rock, 
whether original or produced, is ready to take advantage of any 
crack, whether it be called joint or fault, and may begin to rise, 
exactly as water rises in a fractured sheet of ice nearly to the sur¬ 
face, a relatively large mass of the rock, like the ice, settling a 
small distance, to compensate for the considerable uprise of a 
small amount of liquid material. The rise of ice and lava alike 
are therefore gravitative. Probably, however, or at least pos¬ 
sibly, liquid rock is upon the average somewhat heavier than 
the superjacent solid material, just as water is heavier than 
ice. In this case it would not rise to the surface. However, 
if the solid material were supposed to be in an amorphous, or 
crystalline condition, the change to the liquid condition would 
lessen the specific gravity, for a given mass of liquid rock occu¬ 
pies more space than an equivalent mass of glass of the same 
character (in the case of diabase 3 per cent. 1 ), and in the amor¬ 
phous solid condition occupies more space than in the crystalline 
condition (in the case of diabase about 10 per cent. 1 ) The 
average specific gravity of the known solid crust of the earth is 
about 2.7. That of diabase glass, a rock fully as basic and 
heavy, as the average of extrusives, is 2.717. 1 The specific 
gravity of fused diabase at ordinary pressures is about 2.635. 
Furthermore the development of steam bubbles may greatly lessen 
the specific gravity of the magma. It therefore appears probable 
that upon the average the liquid rock cannot be supposed to be 
heavier than the subjacent solid material. Locally the magmas 
1 High temperature work in igneous fusion and ebullition, by C. Barus: 
Bull. U. S. Geol. Survey, No. 103,1893, pp. 26, 38. 
