446 JOURNAL OF THE WASHINGTON ACADEMY OF SCIENCES VOL. 13, NO. 20 
ereater strength than those segments which affected the two Americas and 
Australia. 
In the opinion of the writer the movements of the continental land masses 
in the Tertiary mountain-making are not in accord with any form of con- 
traction hypothesis, neither with that derived from Laplace, nor Chamberlin, 
nor Wegener, but are in perfect accord with the action of a tidal force. The 
present tidal force exerted by the Moon upon the Earth is calculated to be 
equal to about 1/1,000,000th of gravity—too small, apparently, to be re- 
garded as competent. The problem, then, is to find a competent tidal force. 
For many years the writer has entertained a theory of direct capture for 
the origin of satellites, including, of course, the Moon. The event of the 
Moon’s capture toward the close of the Cretaceous period, and its retention 
as a permanent satellite of the Earth, furnishes the circumstance and a 
competent agency for the production of the necessary tidal force. The 
following considerations are believed to afford adequate support for this 
claim. 
‘“‘The tide-producing force of a body varies inversely as the cube of its 
distance and directly as its mass.’’ (Young’s Manual, 1912, p. 303). ‘The 
Moon’s present mean distance is in round numbers 240,000 miles. At half 
this distance the tide-producing force would be 8 times as strong. At 1/10th 
or 24,000 miles it would be 1000 times as strong. At 1/20th or 12,000 
miles it would be 8000 times as stong. The present tidal force is equal to 
about 1/1,000,000th of gravity. At 24,000 miles it would be 1/1,000th of 
gravity, and at 12,000 miles 1/125th of gravity. 
The eccentricity of the Moon’s present orbit is 0.0549, or nearly twice 
that of any other large satellite. The first fourteen of the short-period com- 
ets in Young’s table (p. 586) have eccentricities ranging from 0.40 to 0.84. 
If the Moon was acquired by direct capture, as here postulated, its eccen- 
tricity must have been much greater at first than it is now. In the absence 
of direct knowledge, it seems reasonable to assume that the Moon’s eccen- 
tricity at the time of capture was of about the same order of magnitude as 
those of the short-period comets or something between, say, 0.50 and 0.90. 
Thus, when the Moon was first captured, the tidal force was, in all probability, 
something between 500 and 3000 times as strong as it is now. With the 
Moon’s perigee 24,000 miles from the Earth, the tidal force would be 1000 
times greater than it is now, and this is certainly a conservative assumption 
for the Moon’s eccentricity at that time, and for its distance at perigee. 
Since capture, the reduction of eccentricity has, of course, been extremely 
slow, capture having taken place at least two or three millions and perhaps 
four or five millions of years ago. 
At the present time earthquakes show well marked maxima and minima 
corresponding to the Moon’s nearest and farthest distances from the Earth 
respectively. This effect is intimately responsive to the varying power 
of the tidal force, and with increasing degrees of eccentricity and decreasing 
distance of perigee it would be rapidly increased under the law stated above. 
The maximum tidal force would, of course, act for only a few days in each 
month, but it would be very powerful and fully competent, in the writer’s 
opinion, to produce the observed results. It would tend to increase by a 
small amount the oblateness of the Earth, and would by so much increase, 
1In a paper entitled Bearing of the Tertiary Mountain Belt on the origin of the 
Earth’s plan (Bull. G.S. A., 21: 179-226. 1910) the writer discusses these and other 
points more fully. 
