401 
water contains carbonic acid gas in solution, as all meteoric water does, 
cavities will be formed in it. 
The regions in which sinkholes occur were originally covered with 
deciduous forests and as a result the surface was covered with decaying 
vegetable matter. It is well known that this condition reduces the surface 
run off and allows more water to sink into the ground. Shaler (91) has 
also shown that this decaying humus produces a large amount of carbonic 
dioxide, so that the water. passing through it, is always saturated with 
this acid. From these facts, it is probable that the formation of caves 
and sinkholes formerly occurred more rapidly than at present. 
What causes a sinkhole to develop at a particular point is somewhat 
conjectural. Something occurs which increases the rate of solution at a 
particular point. There may be a place in the stone which is more soluble 
than the surrounding rock. It has been suggested that fault-lines may be 
the initial cause of at least some of them. ‘There is a fault near the mouth 
of Shawnee cave in the Mitchell limestone but no line of sinkholes has de- 
veloped along it. 
It is quite possible that the tap roots of some of the walnuts, oaks and 
similar trees of the original forests may have determined the location of 
some of these depressions. These tap roots undoubtedly reached bed rock 
in many places. When they decayed they left a funnel shaped opening in 
the soil, filled with their own decaying stems. This funnel would con- 
duct meteoric water immediately to bed rock and charge it with CO* as it 
did so. 
Cummings (05, page 87) explains this formation as follows: 
“Where two joints intersect. the enlargement is apt to be greatest, 
giving origin to funnels, narrowing gradually downward, and showing in 
a beautiful way the formation of sinkholes, which are only such funnels of 
solution grown large.” 
Whatever may initiate this process, after connection is once established 
with a subterranean system, the processes of weathering, erosion, etc., en- 
large the funnel in every direction. The funnel is really a valley whose 
source or upper end is the perimeter of the cone and whose mouth or outlet 
is the opening in the center. The sides of a young sinkhole are usually 
very steep and its area limited, while those of an older one are more gentle, 
with a much larger area. At any stage in the development of a sinkhole, 
[26—26988] 
