268 Pennsylvania before and after the Elevation, etc. (March, 
gation due to contraction in that interval is inappreciable, what 
an enormous time must be allotted to the earlier stages of geo- 
logical history! Even allowing for a greater rate of cooling 
in those earlier days, the time will surpass all that geologists 
have demanded, and be more difficult of admission than the 
contraction here contended for. It is not easy to admit that cool- 
ing, contraction and crumpling have been important factors in the 
formation of the surface of our globe, and at the same time to 
deny that their action has been perceptible or important since 
Mid-palzozoic ages, that is during a lapse of time amounting 
probably to not less than fifteen millions of years. 
Further, it is not impossible or improbable that the facts and in- 
ferences here detailed may be useful in the solution of some diffi- 
cult geological problems, especially regarding the older rocks. 
If mass-motion to so great an extent has taken place in the 
earth’s crust since Paleozoic time began—if the tangential 
thrust has produced lateral movements in the rocks such as 
those here described and others which might have been men- 
` tioned—if strata have slid bodily over strata for great distances, 
and whole counties have been shoved for miles out of their pre- 
vious places, it is obvious that enormous lateral displacement of 
strata must be recognized as a momentous factor in geology, and 
that older beds may be found far out of their expected places and 
` even overlying newer ones. Into this subject, however, I cannot 
now enter, but leave it with the single remark that the greatest 
caution and reserve must be manifested and every element of 
doubt eliminated before we can confidently assert that in regions 
of disturbance the upper is the ater deposit. 
I may be allowed to repeat in conclusion that the inferences 
here deduced do not rest on exact accuracy in the figures em- 
ployed. Were they considerably in excess of the truth the. 
argument would still hold good. Even the half of the amount 
of contraction involved would far surpass-what geologists have 
_ been in the habit of claiming or astronomers of allowing. 
