Side- Light upon' Coal Formation. — Gresley. yy 
sist of ironpyrites, lime, quartz (chalcedonic quartz I believe), 
with more or less brown and black carbonaceous material, on 
careful splitting and breaking up show very considerable 
quantities and variations of plant tissues, besides leaves, fruits 
and seeds. These balls, when less metalliferous and more car- 
bonaceous, have yielded marine shells. The loose, uncom- 
pressed condition of most of these vegetable remains, changes 
from coaly material into almost purely inorganic masses, 
clearly testifying to the condition of tJic vegetable matter of 
the coal-beds containing those nodules after burial and sub- 
sidence: in other words, these balls of lime, silica and iron are 
silent witnesses to the fact that the original thickness of the coal- 
seam was not materially different from its present thickness; 
for these balls are pseudomorphis, so to speak, in the sense 
that the fixed carbon and hydrocarbonaceous substances of the 
original vegetable debris, have been dissolved out and their 
places slowly taken by inorganic or metallic substances, which 
gradually crystallized and became a comparatively compact, 
hard rock. That a further study of these concretionary masses 
must be productive of valuable results in this connection goes 
without saying. My most instructive specimens have come 
from coal in Iowa; and out of these, by grinding, wetting and 
polishing, the best plant tissues found in them have been 
brought into view. Binney, it will be remembered, nearly half 
a century ago in England, sliced somewhat similar, but much 
more calcareous masses from the coal ; and the inferences then 
drawn were that coal was largely composed of plant remains 
the same as those seen in the nodules. 
With reference to the formation of the thin strata of shale, 
so characteristic of the great "Pittsburg" coal-bed, some ac- 
count of which will be found in this magazine (vol. XIV., 1897. 
p 356) : it has since occurred to me that on the supposition that 
the vegetable matter of which much o^ this seam is composed 
were drifted into position from off the land where it grew, and 
if we are to accept the dictum of writers who say that when 
decayed vegetable matter is mixed with fine earthy matter and 
is carried down and comes to rest in large areas of water, the 
vegetable detritus sinks first, then if each separate bench or 
layer of this bed of coal represents (so far as its terranean 
vegetable constituents go,) water-transported materials, may 
