Drainage Changes near Granville y Ohio 159 
j 
ii For simplicity the causes inducing piracy will be considered 
j with glacial and diastrophic forces as quiescent. Three may be 
named: Topography, stratigraphy, rainfall. Each of these will 
be considered alone, disregarding the other factors. 
I Topography. Of two drainage systems separated by a divide, 
! the one lying on the steeper slope has a decided advantange. The 
I' impetus given its waters permits it to cut more deeply and rapidly 
j than its opponent.' The divide consequently migrates; the feeding 
;i areas of the weaker stream are gradually gained and more or less 
of its headwater drainage captured by the stronger. The most 
striking cases of piracy occur when the two contending major lines 
flow approximately parallel. This may conceivably permit the 
sudden capture of almost the whole of the weaker system. When 
the major streams flow in opposite directions from the divide sepa- 
ji rating them,*^ as in the Blue Ridge, the ground is sharply contested 
and the diversion of drainage less evident. 
I Piracy may occur between systems of drainage or within a sys- 
f tern. The same laws are operative in either case, 
i Stratigraphy. Differences of structure and dip in the strata 
{ over which they flow may give one of two streams a decided advan- 
j tage over the other. Thinly bedded strata offer less resistance to 
[ weathering, corrasion and corrosion than do heavily bedded strata, 
f The direction of outcrop relative to stream flow is also a factor in 
erosion. Chemical composition and structure, whether unmeta- 
morphosed sedimentary rock or igneus or metamorphic rock, must 
: also be considered. 
By advantageous combinations of the above one stream may 
j cut its channel more rapidly and eat headward faster than its 
neighbor, thus securing substantially the same conditions as in the 
case of the stream with steeper slope. 
Rainfall. It is evident that, all other conditions being equal, 
of two opposing streams the one in the area of heaviest rainfall 
I would have the greatest advantage. There are many instances 
I where a divide obstructs the prevailing winds causing the precipi- 
1 ration of nearly all their excess moisture on the windward side. 
! In such areas it is evident that the streams draining the territories 
I of greatest rainfall would ultimately gain an advantage similar to 
that favored by topography or stratification. 
® F. S. Mills: Journal of Geology^ vol. xi, pp. 670-678, 1903. 
