OF ARKANSAS. 123 
with some intercalated beds of limestone. The top of this marly shale is 
230 feet below the summit. It is at least 100 feet in thickness, down to 
the junction of this shale and an underlying bed of limestone. 
The rocks are evidently much disturbed in this mountain. At one point 
in the descent, a dip of 4 deg. south-west was observed; but a short 
distance further the dip was reversed. 
CRAWFORD COUNTY. 
+ 
Close to the spring, at the foot of the mountain over which we passed 
from the Barren fork of the Illinois to Lee’s creek, the Archimedes lime- 
stone is in place, 260 feet below the level of the principal mass of corre- 
sponding limestone, in the section of the north-west slope of the mountain. 
There is no doubt a dislocation of the whole of the rocks of the moun- 
tain with a subsidence to the south-east, which causes so sudden a depres- 
sion of this limestone. 
Associated with Archimedes at the spring, near the foot of the mountain, 
on Lee’s creek, occurs Agassizocrinus conicus. 
Lee’s creek meanders for many miles at the base of high cliffs and 
slopes of the spurs of the Boston mountain range, which are composed of 
sandstones, shales, and limestone, belonging to the age of the millstone 
grit and subcarboniferous group, the strata gradually dipping down stream 
towards the south. No coal has ever yet been discovered on this part of 
Lee’s creek, but near its head, 15 miles above Alfred Smith’s place, a bed 
is represented as occurring, and ranging from 10 to 12 inches in thickness. 
Half a mile below Alfred Smith’s farm, sandstone was observed dipping 
10 deg. to the south-east, the Archimedes limestone being no longer visible 
above the bed of Lee’s creek. 
One of the most remarkable features in the scenery of north-western 
Arkansas is the ‘‘ Natural Dam,” represented in the steel plate engraving 
forming the frontispiece to this volume. It is formed by a solid bed of 
sandstone, from 6 to 8 feet in thickness, which runs entirely across the 
bed of Lee’s creek, forming a natural barrier to the descent of the water, 
in consequence of the gradual dip of the rock up stream towards the north- 
east, at an angle of 4 to 5 deg. being just the proper inclination to 
dam the water back, and throw it to a sluice, that might be solidly and 
permanently fixed to this rock wall near where it runs into the north-west 
bank. 
‘The log mill, seen in the frontispiece, is one of the rudest description ; 
