368 The American Geologist. December, 1894 
with a great porous mantle, which readily admits the surface 
waters that trickle down through it until they meet the less 
pervious bed of shale, No. 10, or the sandstone, No. 13. 
The accompanying plate XI represents a typical area in the 
western half of the district. It is a map of Eureka Springs and 
vicinity, drawn to scale from a survey of the area. It shows 
nearly all the constant springs, but omits many smaller ones 
that flow only during a part of the year. The shaded area on 
the map represents the cherty limestone of No. 8, which forms 
the tops of all the hills; the broken line is the outcrop of a 
bed of crystalline limestone ("St. Joe marble") forming the 
base of the chert bed. The black shale No. 10 is present here 
in places as a bed varying from two to five feet in thickness 
and niany of the springs emerge on it. Where it is absent 
or thin the springs emerge on the sandstone No. 13, which 
immediately underlies No. 10. The unshaded part, marked 
as Silurian, includes the sandstone No. 13 and several beds of 
No. 14. 
As may be seen, of the thirty-eight springs on this area of 
five square miles, twenty-six emerge at the base of the chert 
bed, and only twelve find their way through the shale and 
sandstone into the underlying magnesian limestones. Even 
this proportion is much greater in many other parts of the 
area, owing to the thinness of the sandstone bed in this re- 
gion. Where the sandstone No. 13 is thick, practical^ no 
springs emerge beneath it. 
It is noteworthy that this horizon of springs should repre- 
sent the interval between the Silurian and Carboniferous 
rocks over a large part of the area. Nos. 11 and 12 are of Si- 
lurian age, but they occur only in the eastern part of the area. 
The fact that this interval is a marked horizon of springs, is 
probably a coincidence rather than a result of the long time 
interval, as apparently the only effect it might account for 
would be the induration of the sandstone of No. 13. There is 
no unconformity, either of dip or erosion. 
The remarkable features of the region are the great number 
of beautiful springs and the large proportion of them that 
emerge at definite horizons. 
