202 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1923 



velocity of the earth as a whole; but it seems less difficult if the 

 variations are merely superficial, due to the crust sliding non- 

 uniformly on the interior. I have even entertained the wild idea that 

 the motion of the magnetic poles might be due to this cause ; the mag- 

 netism being constant in the interior but with the axis emerging at 

 changing points of the crust as the crust slips over the inner magnet. 

 Unfortunately, so little seems to be known about the motion of the 

 magnetic poles that I have not even been able to make out whether 

 the motion is from west to east, as this theory definitely requires. 



What interests the geologist more nearly is that the brake is applied 

 only at certain areas on the surface, so that there would be a tendency 

 to crumple the crust more particularly to the west of these areas. 3 It 

 is unfortunate that shallow seas are necessarily the least permanent 

 features of the earth; otherwise I would have asked whether the 

 geologists had evidence of special crumpling in such areas. 



I have regarded the crust as fairly mobile from east to west. I sup- 

 pose the geologists would also like it mobile from north to south in 

 order to have glacial periods in those portions which are now near 

 the Equator. It is not possible to hold out much encouragement for 

 such an idea, because we can not imagine any force acting from north 

 to south. Still if the crust, which is being urged by the east-west 

 force of tidal friction, is resisted by obstacles it may be deflected, 

 finding that, say, a southwest track offers less resistance. In a long 

 enough time almost any displacement may have happened, granting 

 my hypothesis that the connection of the crust to the interior is 

 reasonably plastic. So I can not forbid this possible interpretation 

 of glacial periods in the earlier geological times. 



I am sure that it will not be supposed that, in presenting the 

 astronomical side of these questions which belong both to geology and 

 astronomy, I have any intention of laying down the law. The time 

 has gone by when the physicist prescribed dictatorially what theories 

 the geologist might be permitted to consider. You have your own 

 clues to follow out to elucidate these problems, and your clues may 

 be better than ours for leading toward the truth. We both recognize 

 that we are adventuring in regions of extreme uncertainty where 

 future discoveries will probably lead to various modifications of ideas. 

 Where, as in the new views of the age of the earth, physics, biology, 

 geology, astronomy, all seem to be leading in the same direction, and 

 producing evidence for a greatly extended time scale, we may feel 

 more confidence that a permanent advance is being made. Where our 

 clues seem to be opposed, it is not for one of us to dictate to the other, 

 but to accept with thankfulness the warning from a neighboring 

 science that all may not be so certain and straightforward as our own 

 one-sided view seemed to indicate. 



8 1 am not sure whether I am right in assuming here that the frictional brake is applied 

 at the same points of the earth's surface where the dissipation of energy occurs. 



