316 IIk .{linriid II fiioliii/isf . Nuvfiiili.T, IWtt 
v;iii;ititfn> in tlir n-liilivc tliickni'sses of niuiloiroiis lii-ds. and in 
the iigfiregato thicknesses of ditterent cycles. The facts with 
which we are im|)ressed. however, are not the minor discrepancies 
and ap})arent anomalies. Imt the broad lieneral fact of a definite 
sequence. .\nd the conviction Itecomes irresistible that such a 
sequence, must be referred to a periodical recurrence of certain 
fundamental phenonu'ua concerned in the evolution of our planet. 
These phem^mena may l>e considered uiuler two heads in relation 
to this subject : 1st. immediate causes: 2d. remote causes. Tlie 
immediate causes are evidently, (a) the elevation and subsidence 
of hvnd areas, (b) meteoric conditions atfectin<; the rate of erosion 
of these areas (C) the mutual reactif)ns of these sej)arate forces. 
In the remote causes we must seek for an explanation of the 
ptii-ioiHrlti/ of upliea\als and sul)sideiices. It is clear that with a 
ijiven surface of the earth s crust, an elevation of a portion of 
land must either ri'sult in a depression of some other portion of 
land or of tlie sea bottom. Let us supi)ose an elevation of land 
to take place which results only in a depression of the sea bottom. 
There will then In- a smaller surface of the waters of the globe 
exposed to the evaporative influence of the sun. Other things 
e(inal. there will, consequenth'. ]>e a diminution in the amount of 
precipitated moisture. This will necessitate a dimiiuition in the 
amount of erosion and atmospheric disintegration of the emergent 
areas, provided the rainfall l>e efjuable. Hut the amount of 
erosion depends not only ujion the (luantity. distribution and 
periodicity of rainfall, but also upon the angle of sloj)e of the 
land subject to erosion. The etl'ectiveness of erosion A'aries 
directly as sonu- high powt'r of the velocity of the eroding fluid. 
and the velocity is a direct function of the sine of the angle of 
slope. So that the diminished preci})itation due to a diminution 
of the water surface might be more than counterbalanced by a 
disproportionate increase in the angle of slope, in the final results 
of erosion. Let us assume that in a given instance this was 
actually the case. Then the great angle of slope would necessi- 
tate a precipitous fall of torrents. Suppose the slope is such as 
to double the nu>aii velocity of a given volume of water falling in 
a given inteival ot time. Then, whereas formerly this same 
volume of water transported to the sea fragments whost' dimen- 
sions may be represented I ly l''or 1. now it will transport frag- 
nu'uts of 2'"' or ()4. since the transporting power of a Huid varies 
