160 
THE FOSSIL FLORA. 
several reasons for limiting ourselves, in the present article, to this 
district; the first is, it has been far more extensively worked, and its 
productions are, consequently, better Jvnown than anv other. It has, 
also, furnished us with a very large portion of the materials we have 
hitherto made use of; and the residence of one of the Authors in the 
midst of it, has necessarily brought the circumstances attending it 
more particularly under our notice. There is a convenience, also, 
in thus limiting our references, as our observations cannot occupy a 
large space; besides which, we are convinced, that, in every essen¬ 
tial circumstance, the history of one series of Coal measures is the 
history of every other of the same age. 
It was our wish to have appended to this a Catalogue of all the 
vegetable fossils hitherto discovered in it; but, in attempting to form 
one, we have immersed ourselves in a labyrinth of difficulties, one 
half of its fossils having never been described ; and, although we 
could easily ally a portion of these to known genera, yet the greater 
number of them would remain absolute riddles—-waiting for some 
fortunate discovery by which they are to be connected with fossils 
already known, or proved to belong to others yet to be discovered. 
The beds usually denominated the Coal measures, being the 
higher part of the Carboniferous formation, occupy a large portion 
of the Counties of Northumberland and Durham, reposing upon, 
and being conformable to, the inferior members of the series. They 
consist of irregularly alternating beds of sandstone, shale, or argilla¬ 
ceous schist, and coal, whose aggregate thickness may be estimated 
at 300 fathoms. This may not be correct, but is, probably, near 
enough the truth for our purpose. 
With the exception of the coal itself, and a few layers and nodules 
of clay-iron-stone, embedded in some of the shales, the whole of 
these beds are of mechanical origin, the shale being evidently lami¬ 
nated clay, or mud, consolidated by pressure; and the sandstones 
abraded Quartz, Felspar, and Mica, agglutinated by an argillaceous 
or calcareous cement. From whence the immense mass of travelled 
matter, of which these sandstone and shale beds are composed, may 
have come, it is somewhat difficult to conjecture. The sandstones of 
the series below the Coal measures, denominated millstone grit, con¬ 
tain interspersed masses of water-worn quartz, of considerable size ; 
and rarely amongst those of the Coal-formation, a bed will be found, 
partaking of the same characters; but the mass consists of minute 
siliceous grains, which are not rounded, or but partly so; from 
which it is fair to infer, that, whatever were its origin, the sand of 
which they are composed was not brought from anv great distance, 
