Page:The Journal of geology (IA journalofgeology21894univers).pdf/193
succession. The faunal and floral features are practically identical from one end of the great coal basin to the other.
The greatest interest, economically and geologically, in the Carboniferous basin centers around the coal deposits. Their disposition perhaps comes first in importance, both from the standpoint of the operator and of the geologist. The subject is certainly one of wide-reaching import.
There is now abundant evidence to show that the Western Interior coal field at the time of deposition was for the most part a broad, shallow basin opening to the westward into the great continental sea which then occupied most of what is now known as western North America. That the Coal Measures of the region were laid down during a period of gradual, prolonged, though often checked, subsidence is evidenced by all stratigraphical and lithological details, as well as by the characteristic faunal peculiarities. That the coal beds originated largely in coastal swamps of limited breadth but, with some interruptions, of very considerable length, stretching out near sea level for long distances and sending out minor extensions into the old rivers and estuaries is fully warranted by the facts disclosed everywhere. On the low, slowly sinking shores there prevailed at certain times a similarity of physical conditions especially favorable to coal formation. During these intervals unusual amounts of coaly material were allowed to accumulate and to be preserved in places, the period being preëminently one of coal growth, at least for a given province. The great stratigraphic plane marking each record may be appropriately termed a "Coal Horizon."
In stratigraphy, a geological horizon is a level recognizable over a considerable geographical extent, having a more or less well defined stratigraphical position, distinctive as to lithological features and characterized by a particular set of fossils. The term in a broad sense is almost equivalent to formation, and has been used as indefinitely. In its more limited meaning it is applied properly to a minor part or zone of the smallest stratigraphical unit having a commonly accepted specific name. Un-