78 LIMESTONES OF RED CEDAR, 
more than sixty or seventy feet ; while the depth of the Old Red Sandstone of Great 
Britain is said, at certain localities, to equal the elevation of Mount Etna above the 
sea, reaching from ten to eleven thousand feet. Its average thickness, in the 
British Islands, is laid down, by some authors, at about half that amount. 
In the northern part of Iowa these limestones are seldom seen, by reason of the 
extensive drift, except in the deep cuts of the streams, and then only in low mural 
exposures, an example of which is shown in the middle ground of this landscape, 
looking over the extensive prairies in the Valley of Shell Rock, one of the eastern 
branches of the head waters of Cedar River. 
LIMESTONES OF SHELL ROCK, CEDAR VALLEY. 
SECTION II. 
ITS PALAONTOLOGY. 
PALZONTOLOGICALLY, the limestones of this pe 
may be divided into 
1. Lower coralline beds; 
2. Shell beds; and 
3. Upper coralloid limestone. 
The beds containing the greatest abundance of Brachiopodes being included be- 
tween beds charged with fossil corals; of which coral beds the lower is so complete 
riod, in Cedar and Towa Valleys, 
