210 The American Geologist. April, 1905 
solar nebula it is assumed (1) that the central mass was rela- 
tively very great^ (2) that the knots were very irregular in size 
and placed at irregular distances from the center; and (3) that 
the nebulous portion was very small relative to the central mass 
and probably large relative to the knots. 
* * * Since all the planetesimals and planetary nuclei 
were revolving in the same direction about the solar mass, the 
collisions were all overtakes, and could have been violent only 
to the extent of their differences of orbital velocity, modified by 
their mutual attractions. These velocities are of a much lower 
order than the average velocities of meteoritic collisions. Many 
of the overtakes would obviously be due to differences of veloc- 
ity barely suflBcient to bring about an overtake. When the rela- 
tive mildness of impact is considered in connection with the in- 
tervals between impacts at a given spot, the conviction can 
scarcely be avoided that the surface temperature would not neces- 
sarily have been high. It seems probable that it would 
have been moderate throughout most of the period of aggrega- 
tion, and certainly so in the declining stages of infall. * * * 
By graphical inspection of all probable cases, it may be seen 
that the possibilities of overtake favorable to forward rotation 
exceed those favorable to retrograde rotation. This holds true 
on the assumption of an equable distribution of planetesimals, 
which may fairly be assumed as an average fact, but not neces- 
sarily as always the fact; and hence the conclusion is not rigor- 
ous, and a backward rotation is not impossible. From the nature 
of the case, a varying rotation for the several planets is more 
probable than a nearly uniform one. 
It is also obvious that the impacts on the right and left sides 
of a growing nucleus, as well as those on the outer and inner 
sides, might be unequal, and hence obliquity of rotation of vary- 
ing kinds and degrees might arise. As the solar system presents 
these variaticns, the method of accretion here postulated seems 
to lend itself happily to the requirements of the case. 
* * * A planetary nucleus gathers planetesimals that have 
orbits both larger and* smaller than itself, and hence in effect it 
sweeps a space both outside and inside its own zone The 
breadth of this space is dependent on the eccentricity of its own 
orbit and on the eccentricities of the orbits of the planetesimals 
it gathers in on either hand. 
* * * For the large planets that have dominated their col- 
lecting zones and presumably swept them thoroughly, the reduc- 
tions of eccentricity are subequal. For the very small bodies that 
presumably grew but little, the eccentricities remain large, for 
the greater part. For example, the eccentricity of Mercury, the 
smallest of the planets, remains more than twice that of any 
other planet. Mars, the next smallest in size, comes next in ec- 
centricity among the planets, while the asteroids, which probably 
