•^^"2 Ilii Aiiiirnan Gt(>l<><jixf . n.ivi-iiiImt, ih!u 
this as it may. tli(M»' seems to be irood reason foi' ref»'rriii<; gen- 
eral and widespread movements oeenrrinii durin<r protracted in- 
tervals, like those wiiicii took phice on all the continents durinn' 
the rarljonit'erous a<i'e to «ieneral canses. We have stated before 
that I>r. ('roll refers the movements of this epoch to alternate 
periods of refi'iu'ei'atioii on the northern and sonthern hemi- 
spheres, snch periods favorinu' tlie formation of an ice caj). and 
tluis. by chanii"in<>' the earth s centi'e of gravity cansinii' a rise of 
waters on the i-efrigerated hemisphere. No donlit this nniy ex- 
plain the fact of differences of consecntive strata, but it <loes 
not solve the proltlem of cycles of sedimentation, and it appears 
to me thai a diminishinu' angle of slope in the wasting land nnder 
nniform conditions of precipitation is the oidy ade(puite solution. 
But even this does not explain those gianderdivisions of geologic 
time like the l*al;eo/.oic. Mesozoic. and Caeno/.oic. We may 
suppose that movements liki' the uphea\al at the close of the 
Lower Silurian, in the Permian, or in Tertiary times were either 
the expenditures of accunnUated tensions created by contraction.'^ 
in tlie earths crust, or a contemporaneous action of this with one 
or more of the other forces enumerated hei'etofore. 
Snch a periodicity of major events separated Ity long intervals of 
time and interspersed by minor commotions liannonizes with the gen - 
»M'al hypothesis of a cooling and contracting globe. We would 
infer thai in early times when the crust was thin, the ra<liation of 
heat was much mori' active than now. and general, peripheral 
convulsions were of frecpient occuri'enci'. J?ut owing to the thin- 
ness of the ciiisl it t'ould not sustain any very marked elevations 
without a lupturi' and outflow of molten matters witliin. Sucli 
outflows, if copious, would olditerate. any existing ditferences of 
level, and I'cfnse the pai'ts of llie crust which wei'e crumpled in- 
ward. This i)rocess would of neci'ssity continue until the crust 
should have attained a snUicient thickness to sustain the eleva- 
tions impressed upon it. Assuming that this was the ordei' of 
events we would expect that eh'vations would take place ak»ng 
lines of weakness (see'n//r): that tlu' major contractions would 
be co-simultani'ons on ditfei'ent parts of the glolie. that tin' older 
mountains would have a smaller elevation than tliose that were 
formed later: and. linally. inasmuch as the rate of radiation of 
internal heat sustains an inverse ratio to the thickness of the 
crust, whereas the resistance to tlexnre sustains a dii'ect I'atio to 
