Physical Tkeorie,^ oj the Earth— Reade. lOfi 
The existence of a level-of-no-stra,in though most important 
was only one of the objections I had to urge against the accept- 
ance of the contraction theory as an explanation of the orij^an 
of mountain ranges. It may not be unprofitable for me to 
summarize these objections, that those American geologists 
who have the opportunity and desire may test t]ie ideas in the 
field. 
One of the great arguments formerly relied on by the sup- 
porters of the contraction theory was the enormous amount of 
lateral compression which many mountain ranges had under- 
gone. 
This in the case of the Alps is estimated by Helm to show 
a shortening of 72 miles, and according to Prestwich, Prof. 
(Jlaypole estimates the linear compression of the Appalachians 
at 88 miles. These of course aie only meant as approxima- 
tions, but I contend that the system of measurement is falla- 
cious though I accept it for the purpose of illustration.* 
The only possible way of accounting for siK'h extensive 
movements was by a shortening of the earth's i-adius, which 
of course any theorizer is at liberty to do for himself on any 
scale he pleases so long as he does not work on known data. 
This shortening given and a crust in compression of the requir- 
ed thickness, the whole phenomena of mountain ranges are sup- 
posed to be acc^i^unted for. Let us see what such an hypothesis 
involves. Taking for example the Appalachians; as the whole 
of the strata from 8 to 10 miles thick from base to summit are 
said to be practically conformable, the shrinking of the earth 
cannot have affected this area from the commencement of the 
Cambrian period to the close of the Carbonifei'ous, a space of 
time in which is comprised a considerable portion of the geo- 
logical history of the globe. The Triassic rocks are unconfor- 
mable to the Carboniferous, so that the main elevation of the 
Appalachians must have taken place between the latter part of 
the Carboniferous and the beginning of the Triassic periods. 
To this s})ace of time then, we must perforce limit the trans- 
verse shortening of the strata. If correctly estimated at 88 
miles and the whole were the effect of the earth 's contraction 
it would, on the highly favorable assumption that the whole of 
the linear circumferential contraction was disposed of on this 
♦This is explained in the "Origin of the Mountain Ranges." 
