GEOLOGY: B. WILLIS 
377 
This investigation has suggested a possible physical cause for the 
position of the ecliptic plane which, so far as I am aware, has not been 
satisfactorily accounted for. It is purely speculative and I know of no 
test which would be adequate, particularly as it rests upon the theory 
of capture which is far from being established. This explanation was 
suggested by the fact that the ecliptic intersects the galaxy not far 
from the extremities of the axis of preferential motion. There is some 
evidence that the two star streams may be in reality parts of a curved 
stream, the so-called axis of preferential motion being in reahty a 
tangent to that curve. In my opinion such a conception best satisfies 
the observed conditions. Radial velocities indicate maxima to the 
south of both extremities of the so-called ellipsoidal axis. This brings 
the intersection near 18^ in the general region of what appears to be 
one star stream. 
The centers of preference of the paraboKc comets near the longitudes 
of the apex and antapex of solar motion are also both some 10° or 15° 
south of the ecliptic. If now the planets were captured, it is not dif- 
ficult to outline a cause for the position of the ecliptic. Bodies moving 
with a stellar stream (not to be more definite at present) might be picked 
up by the Sun and if acted upon by a resisting medium, have their orbits 
rendered nearly circular. By mutual perturbation their orbits would 
tend to assume a common or 'invariable plane.' The addition of the 
masses of the satelHtes, comets and asteroids would not greatly disturb 
this plane, once established. This accounts for only one coordinate 
of the plane. The other we must suppose to be the mean of the group 
and as far as we know, at present, accidental. 
This hypothesis is admittedly speculative and further discussion at 
this time does not seem warranted. 
DISCOIDAL STRUCTURE OF THE LITHOSPHERE' 
By Bailey Willis 
Department of Geology, Leland Stanford University 
Communicated by J. M. Clarke, July 15, 1919 
Underlying the thesis here presented are the following postulates : 
a. The lithosphere is heterogeneous as to density; 
h. The lighter and heavier masses are of notable dimensions, varying 
from a hundred to several thousand kilometers in diameter; 
