564 ll... €. CHAMBERLIN 
activity. It is established by the most ample observation in 
mining, that below the permanent water level disintegration has 
made slow progress compared with that in the zone above the 
water level. Now the thickness of the zone between the surface 
and the permanent water level is intimately dependent upon the 
general altitude. Ina continent reduced approximately to base 
level, this zone is exceedingly thin. Ina region much elevated and 
deeply dissected by erosion, the thickness of the zone is very 
much greater. As between a continent with an average eleva- 
tion of 2000 feet, at the climax of dissection following a crustal 
readjustment, and a continent of similar area with an average 
elevation of 300 feet, during a period when it is approximately at 
base level, the average depth of the aérated zone above the 
water level, probably varies more nearly with the square of the 
elevation than as a direct multiple of it. It is improbable, how- 
ever, that the chemical action is augmented at so great a ratio. 
Probably greater warmth and more abundant vegetation are 
correlated with the lower altitudes, and both these aid chemical 
action. This in turn is somewhat offset by the greater mechan- 
ical disaggregation which results from changes of temperature 
and from gravitative influence in the more elevated condition. 
Making all allowances that seem required for the offsetting 
factors, it would still appear that the elevated condition 
increases the activity of decomposition in a very notable degree. 
Below the permanent water level, the advantage probably 
also lies greatly with the higher elevation. The action of sur- 
face water upon the deeper rock is dependent upon the unbal- 
anced hydrostatic pressure which promotes underground 
circulation and forces the water through the crevices and pores 
of the rock. If the underground water stands near sea level, 
there is little unbalanced hydrostatic pressure to promote active 
circulation and thereby carry the surface waters, enriched with 
atmospheric gases, down into the lower strata and bring them 
into action. The atmospheric waters precipitated upon the 
surface run away chiefly at the surface, and fail of the contact 
necessary for action. On the other hand, in an elevated region 
