PERMIAN AGE AND CLOSE OP THE PALEOZOIC. 165 



raolfcen rocks spewed out to the surface from tlie 

 softened and molten masses below. Fourthly, the 

 basin of the Atlantic must have been sufficiently strong 

 to resist the immense lateral pressure, so that the 

 yielding was all concentrated on the weaker parts of 

 the crust near the old fractures at the margins of the 

 great continents. In. these places also, as we have 

 seen in previous papers, the greatest thickness of 

 deposits had been formed; so that there was great 

 downward pressure, and probably, also, greater soften- 

 ing of the lower part of the crust. Fifthly, as sug- 

 gested in a previous chapter, the folding of the earth's 

 crust may have resulted from the continued shrinkage 

 of its interior in consequence of cooling, leading after 

 long intervals to collapse of the surface. Astronomers 

 have, however, suggested another cause. The earth 

 bulges at the equator, and is flattened at the poles in 

 consequence of, or in connection with, the swiftness 

 of its rotation ; but it has been shown that the rotation 

 of the earth is being very gradually lessened by the 

 attraction of the moon.* Pierce has recently brought 

 forward the idea f that this diminution of rotation, by 

 causing the crust to subside in the equatorial regions 

 and expand in the polar, might produce the move- 

 ments observed ; and which, according to Lesley, have 

 amounted in the whole course of geological time to 

 about two per cent, of the diameter of our globe. We 



* Sir "William Thomson, who quotes Adams and Delaunay. 

 t " Nature," February, 1871. 



