se commercial importance of the
coal seams, it became the practice to distinguish a "productive"
(_flotzfuhrend, terrain houiller_) and an "unproductive," barren
(_flotzleerer_) Lower Carboniferous; these two groups correspond in
North America to the "Carboniferous" and "Sub-Carboniferous"
respectively, or, as they are now sometimes styled, the
"Pennsylvanian" and "Mississippian." But it was soon discovered that
the "productive" beds were not regularly restricted to the upper or
younger division, and, as E. Kayser points out, the real state of the
matter is more accurately represented by the subjoined tabular scheme.

+-------------+-------------------+------------------------------+
| | Continental Type | Marine Type of |
| | of Deposit. | Formation. |
+-------------+-------------------+------------------------------+
| | |Younger Carboniferous |
| Upper |Upper _Productive_ | limestone and the _Fusulina_ |
|Carboniferous| Carboniferous | limestone of Russia and |
| | | Western North America |
+-------------+-------------------+---------+--------------------+
| Lower |Lower _Productive_ | Culm |Lower Carboniferous |
|Carboniferous| Carboniferous |(in part)| limestone series |
+-------------+-------------------+---------+--------------------+

While the continental type of deposit, with its coal beds, was the
earliest to be formed in certain areas, and the marine series came on
later, in other regions this order was reversed. It should be
observed, however, that the repeated intercalation of marine deposits
within the continental series and the frequent occurrence of thin
coaly layers in the marine series makes any hard and fast distinction
of this kind impossible.

The so-called "unproductive" or barren strata, that is, those without
workable coals, are not always limestones; quite as often they are
shales, red sandstones and red marls.

In subdividing the strata of the Carboniferous system and correlating
the major divisions in different areas, just as in other great
systems, use has to be made of the fossil contents of the rocks;
stratigraphical units, based on lithology, are useless for this
purpose. The groups of organisms utilized for zoning and correla