270 THE EVOLUTION OF CONTINENTS. 
greatest minimum when what is now the eastern conti- 
nent came under his eye; then, as the Atlantic basin 
swept along, there would be asecond but smaller and 
briefer maximum, itself followed by asecond minimum 
as the non-luminous spot which we now call the western 
continent swept into view. This seems almost like the 
description of 6 Lyrae (I quote from Vewcomb’s Astrono- 
my, page 429). ‘*That star has twomaxima and minima 
of unequal brilliancy. Should we observe it to-night, 
we might find it of the four and one-half magnitude ; 
then in the course of three days it rises to the three and 
one-half magnitude. It then begins to get smaller, and 
comes down to the fourth ; once more it begins to grow 
brighter, and rises to three and one-half, and again it 
diminishes to its first size. The whole variation re- 
quires thirteen days, and is divided into two unequal 
portions of six and seven days each.’ In reference 
to the variations being caused by dark stars, he said: 
‘‘In Algol there seems to be only one spot. About once 
in three days it fades from the two and one-half magni- 
tude to the fourth, and remains there afew hours. This 
change cannot be due to a dark star passing between us 
and Algol, for the latter does not loseits light and re- 
gain it in the same time, which it would do if the 
change was caused in that manner.” 
What may be the real cause of the changes in the va- 
riable stars I am unable to say. Probably in general, 
they are due to spots such as those seen on our sun, yet 
the great persistence of some of them suggests some- 
thing different, as the greatest duration of solar spots is 
only a matter of a few weeks—possibly months. 
The moon and Mars are relatively near us, and it is 
possible to study their features. It may be said that as 
neither presents features such as the earth, although 
they, too, have passed from the molten to the solid — 
ales 
