[114] At the South Joggins, in two or three cases, beds of bituminous shale full of Naiadites and Cyprids have by elevation and drying become fit for the growth of trees with stigmaria roots; but this is quite exceptional, no doubt arising from the accidental draining of lakes or lagoons on their elevation above the sea level.
Some of the thinnest coals, and some beds so thin and impure that they can scarcely be called coals at all, are the most instructive. Witness the following from my section of the South Joggins.
Coal Group 1, of Division 3, is the highest of the series. Its section is as follows:—
"Grey argillaceous shale.
Coal, 1 inch.
Grey argillaceous underclay, Stigmaria.
"The roof holds abundance of fern leaves (Alethopteris lonchitica). The coal is coarse and earthy, with much epidermal and bast tissue, spore cases, etc., vascular bundles of ferns and impressions of bark of Sigillaria and leaves of Cordaites. It may be considered as a compressed vegetable soil resting on a subsoil full of rootlets of Stigmaria." In this case the coal is an inch in thickness, but there are many beds where the coal is a mere film, and supports great erect stems of Sigillaria, sending downward their roots in the form of branching Stigmariæ into the underclay, thus proving that the Stigmariæ of the underclays are the roots of the Sigillariæ of the coals and their roofs.
Here is another example which may be called a coal group, and is No. 11 of the same division:
"Grey argillaceous shale, erect Calamites.
Coal, 1 inch.
Grey argillaceous underclay, Stigmaria, 1 ft. 6 in.
Coal, 2 inches.
Grey argillaceous underclay, Stigmaria, 4 in.
Coal, 1 inch.
Grey argillaceous underclay, Stigmaria.
"This is an alternation of thin, coarse coals with fossil soils. The roof shale contains erect Calamites, which seem to have been the last vegetation which grew on the surface of the upper coal."
Such facts, with many minor varieties, extend through the whole eighty-one coal groups of this remarkable section, as any one may see by referring to the paper and work cited in the preceding note. It is possibly because in most coal fields the smaller and commercially useless beds are so little open to observation, that so crude ideas derived merely from imperfect access to the beds that are worked exist among geologists. The following summary of facts may perhaps serve to place the evidence as to the mode of accumulation of coal fairly before the reader:—