But against these and other few cases of marked identity of the plants of certain beds of coal, it is right to say that generally the data furnished by remains of fossil plants have proved to be either insufficient or deceptive. Insufficient, because the specimens of fossil plants are rarely found representing such a number of species that a satisfactory comparison may be made between the plants of different localities;—deceptive, because the vegetation of the coal period like that of the present epoch has been subjected to great variations, according to geographical distribution or local influences. Therefore, the plants found in connection with two beds of the same horizon may be far different in types, especially when observed at distant localities. And per contra, it has been seen also that sometimes two beds of coal at different levels have been formed of plants of closely allied even identical species.
What it is until now possible to clearly see, in the vertical distribution of the plants of the coal measures, is that the types of the pre-carboniferous and carboniferous flora have been subject to four distinct modifications of their essential characters, and that therefore the series is composed of four distinct groups generally marked upon the whole extent of the formation.
The first comprising the lower or pre-carboniferous, is persistent in the Devonian, the Chemung, and the Catskill.
The second, partly marked in the Pocono, becomes modified and persistent in the sub and intra conglomerate measures.
The third pertains to the lower productive coal measures from above the conglomerate to the base of the barren beds of Pennsylvania.
The fourth is that of the upper productive coal measures, including the Pittsburgh coal and a few hundred feet above. Of course, some species of each group have a greater degree of persistence and pass from a lower to an upper stage.
In the 2nd volume of this [Coal Flora - GL,III, ed.] Work, p. 669, Chapter VI, I have said, on the stratigraphical distribution of the coal plants most of what is known until now on a general point of view. The remarks have been derived from a table of distribution which was composed from the materials I had then obtained. Now, though I have nothing to add to those remarks, a number of new species having been described since in the third volume, it would be advisable to have them inserted with the others, in a table of the same kind, for future comparison.
But the first table, if it does represent well enough the stratigraphical distribution of most of the species, does not take into account all the localities where the plants have been found, and this is a very important matter to be considered, the localities giving, by the number and characters of their plants, fixed points for comparison and researches.
As it is not possible to name all the localities upon a table like the one formerly made, I have prepared, to close the volume, a simple table of distribution by a number of columns about like those of the first, indicating the geological horizon either known or supposed where each species has been recognized.
The number of places where a species has been found is merely marked by numbers in each column.
Besides this I have made lists of all the species of coal plants with the names and horizons of the localities where each of them has been found.
The table, therefore, and the enumeration of the plants will afford the means of comparing both the geographical and stratigraphical distribution of the plants, while at the same time, they give, taken together, a clear exposition of the vegetation of the carboniferous period of the United States, as far as it is known to the present time.
The present Table of Distribution is composed of 17 columns.
[The following pages are presented in pairs in facsimile, as optical character recognition has its limitations - GL,III, ed.]