SOUTHERN CALIFORNIA ACADEMY OF SCIENCES 39 
The density increases with the depth, and it is estimated that the 
average density of the interior will equal two and one-half times that of 
steel. 
The earth acts like a solid body, and not like one of liquid interior, 
It is held that the rock forming the central portion has a melting 
point that varies with the pressure, just as water under different degrees 
of pressure has a different boiling point. 
According to this theory, the earth’s core could easily be solid, and 
the densest portion of its mass. 
The central rock-matter is complex, not simple. Under the intense 
heat and pressure that prevails in the interior, all earth material has 
been fused and squeezed into a conglomerate mass. 
The land formation on the earth’s surface is due to movements in 
the outer crust, producing superficial unevenness. Were the earth a 
perfect sphere, it would be covered with a layer of water about two 
miles deep. 
Three theories are given to account for the solidifying of the earth: 
First. The nebular hypothesis, which teaches that the planets and 
satellites having been successively thrown off from one great central 
nebulous mass, condensed from nebulous rings into globes, and solidified 
gradually by cooling on the surface; the cooling process gradually ap- 
proaching the center, leaving the earth composed of « hard crust of 
uncertain depth, enveloping a molten interior. 
Lord Kelvin has estimated that according to this theory it would 
have required six hundred million years for the earth’s crust to have 
solidified to the deptk of twe or three miles: The theory is not generally 
held by geologists. 
Second. The solidification from the center outward, caused by the 
intense pressure, which is greatest at the center, and which greater pres- 
sure would also be the cause of greater heat at the center. 
Third. The ‘‘planetesimal’’ theory of Prof. Chamberlain of Chicago 
University, which holds that nebulous matter revolving about orbital 
centers collects other similar material, revolving about the same or a 
neighboring center, and in the course of time a planet or satellite is thus 
formed by accretion from without. 
‘According to the first theory, mountains are formed by a contrac- 
tion of tne outer crust due te cooling, and vroleanees by the expansive 
force of the inner heat, breaking through the crust at the weakest 
points, and giving vent to the molten mass within. 
This theory, the speaker thinks, does not sufficiently account for the 
deep-seated forces necessary to fold the basic rock layers of mountain 
ranges into the crumpled condition so commonly known to. exist. 
He leans to the general reasonableness of the theory which supposes 
the solidification to move from the center outward. 
This would force gases and liquids to the surface, where we find 
them, and might afford a rational explanation of much of geological his- 
tory difficult to understand. It also appears to account fairly for. the 
phenomena now recognized as radium emanations. 
Actual eraters on the earth are very small compared with the area 
covered by the mass of material thrown out of the crater. 
’ +The reverse seems to be the case on the surface of the moon, 
where the opening of the craters seems to be more extensive than the 
rims formed by the eruptions.. 
