ol8 Jill Alii<ri<il II (iinlix/isf. Niivi-iiiIm'i. IKiil 
Mssuuu'd ;i uiiUonnily of im'ti-oiic coiKlitioiis. Imt Midi :i iiiiil'orin- 
ity is im|«)ssil)U' from tlu' vcrv luiturc of tlic other coiKlitioiis 
assunu'd. For wIumi the land has a iircat cU'valion the iiioistiuf 
privipitatcd upon it will assiinic the form of snow and ice which 
will inauii'nratc \arions climatic chan<r('s. As the land is worn 
away then' will Ik* a greater surface of water exposed to evapora- 
tion. :ind this will react ui)on the ({iiantity of preci|)itation. Auain 
the disaj)pearance of land will allow oceanic currents to pursue 
new channels which will directly react ui)on climate and indirectly 
upon precipitation. Finally the land will in most cases lie suliject 
to oscillations of lexcl which will react ni)oii climate in a complex 
and indirect way. and upon the rate of sedimentation in a direct 
way. Any one of these variations will interfere with tlu- uniform- 
ity of the cycle, and when several of them are concerned simul- 
taneously it ^vill he correspond iuirly comi)lex. So much for the 
immediate causes. In the remote causes we must seek for an 
explanation of the iHriDiliclti/ of upheavals and snltsideiices 
which have oiven rise to cycles of sedimentation. 
For our present purposes it will not he necessary to enter iiitct a 
discussion of the causes which have determined the existini: con- 
figuration of land areas. Without doul>t. Prof. Darwin's appi'al to 
astronomical influences suggests a general explanation of the leading- 
facts. Also |)i'. A. Winchell shows (World Life) how the hmar 
tides during the earth s early incrustive stages would imi)ress 
meridional lines of weakness. The suhsidence of the earth's 
equatorial protulierance diu- to the secular diininiition of its lota- 
tional velocity would c<msi)ire to a like result. When such lines 
i>f weakness are once estalilisheil every contraction of the crust 
due to the radiation of inti'nuil heat will give rise to rugosities 
along those lines, and these rugosities once established will tend 
to grow with every increase in the lateral pressure. So much for 
the generalities. lint there are certain sjx'cial and very remark- 
able phenomena which these hypotheses do not take into account. 
Such are the j)eriodicities of upheaval and subsidence which have 
given rise to cycles of sedimentati(»n. >[allet has aigued in 
another connei-tion that tlu' contraction of the I'arth's crust dueto 
secular cooling would liy the crushing <»f the rocks give rise to an 
enormous (piaiitity of heat, and that this crushing would take 
place after long intt'rvals of (|uiet. during which intervals the 
tension lU'cessarv to produce the crushing would be generated. 
