INTRODUCTION. 
59 
By tracing the direction of these geological formations on the east 
and west, we cannot fail to observe a convergence to the southward; 
and it is there, if at all, that facts are to be obtained that will show 
conclusively the relative age or synchronism of these formations lying 
between the Coal measures and the Chemung group. 
At the commencement of the Coal-measure deposits, we find a return 
to those conditions which have prevailed during all the preceding 
important sedimentary groups. This formation exists in Nova-Scotia 
and New-Brunswick, and again in Pennsylvania, stretching southward 
into Alabama; while to the west it extends with slight interruptions, 
but with diminishing thickness, far beyond the Mississippi river. 
The thickness of this formation in Nova-Scotia, according to the 
careful measurements of Sir William Logan, is more than fourteen thou¬ 
sand feet. In Pennsylvania, Professor Rogers states it, including the 
conglomerate, at more than eight thousand feet*. Taking these as 
initial points, and carrying our observations to the west and southwest, 
we find a constantly diminishing thickness in that direction, and finally 
the entire formation is represented by a few hundred feet of strata. 
The measurements of Professor Swallow, in Missouri, give six hun¬ 
dred and forty feet as the thickness of the Coal measures proper; and 
in Iowa the amount is still less. 
The great Conglomerate which lies at the base of the formation, and 
in Pennsylvania measures fourteen hundred feet, appears in considerable 
force in Ohio and in Indiana, but has entirely thinned out before 
reaching the Mississippi river. Of the other members of the formation, 
the coarser materials have diminished, and the Coal measures in the 
west are composed of finer sediments than in the east. 
Thus we see everywhere the operation of the same law, viz. a greater 
accumulation, and a coarser character of sediments along the line of 
the Appalachian chain, with aL gradual thinning to the westward, and 
a deposition of the finer or far transported matter in that direction. In 
# Even admitting, as has been claimed by some, that this thickness is over-estimated, a difference 
of one or two thousand feet would not affect our general conclusions. 
