Clastic sediments are less abundant and there are fewer breaks in the succession.
- The system is composed chiefly of clastic rocks, and their composition and structure show that the water in which they were deposited was shallow.
In the Mississippi Basin the larger part of the system is of limestone, though there is some clastic mateiial in both its basal and its upper parts.
The Pottsville formation is chiefly clastic, and corresponds roughly to the Millstone Grit of England.
Swauk formation (clastic rocks with some tufT, &c.) 2005000
20 it occurs with zinc in reticulated deposits and fissure veins in clays and clastic limestones; and in the third, of which Jasper county is much the most important county, the two metals occur in pockets and joints in the Burlington-Keokuk beds of the sub-Carboniferous.
Here clastic rocks predominate, while limestone is more abundant in the interior, If the maximum thicknesses of all Devonian formations be added together, the total for the system is as much as 15,000 ft.; but such a thickness is not found in any one pluce.