THE LOWER COAL MEASURES. 137 
At this time the floor of the gulf seems to have been brought up 
nearly to the sea level over a wide area, and it seems probable that 
numerous islands appeared throughout this area. Around their shores 
as well as around a main coast line, the coal-forming swamps could 
grow. ‘The instability and frequent interruptions of the Freeport coals 
can be understood from this point of view. They nowhere show the 
continuity of the lower seams. They occur in basins, and not in con- 
tinuous sheets, with a greater breadth of outcrop than the seams below 
them. They suggest an archipelago rather than a continental margin. 
But this period was terminated by the return of the sea in full force. 
The Mahoning sandstone, coarse and often conglomeratic, makes the 
roof of the Upper Freeport coal, and it is to the strong invading cur- 
rents that brought in the sand and pebbles of this formation that the 
frequent and often disastrous “wants” are due, which occur even in 
the districts where this seam is at its best. | 
These are theoretical questions, but they bear directly on practical 
and economic interests. The amount of coal that the Ohio coal field 
contains turns, of course, upon the extent of the seams. If we see 
reason to believe that these lower seams originated in marginal swamps, 
with the sea near at hand, then of course we abandon the older view 
that the coal seams extend indefinitely towards the center of the basin. 
A breadth of a few miles, of a score or so, at most, would be all that 
could be reasonably expected for any such seam. ‘The earlier and the 
later seams could no longer be looked for in the same section, and 
instead of concluding that the amount of coal, underlying any given 
county or town in the coal area, depends on its proximity to the center, 
and deepest portion ot the basin, it would be nearer the truth to make 
the amount of coal inversely proportional to such prozimity. In other 
words, we should expect to find the interior of the basin filled with 
“terrains mort”—that is, with dead or unproductive rock. 
| Almost all of the facts that have a bearing on the question seem to 
support the view that is here presented. 3 
Not only is there no instance known in the State in which the 
Sharon coal is mined under the outcrop of the Kittanning coals, for 
example, but there is no instance known in which the Sharon coal has 
been found of mineable thickness directly under a mineable thickness of 
the Kittanning coals. Not a year goes by which does not bring numer- 
ous tests of the facts involved in these statements in the shape of drill 
