The Geological History of Plants

by Sir John William Dawson

Large woody trunks, carbonised or silicified, and showing wood-cells with hexagonal areoles having oval pores inscribed in them, occur abundantly in some beds of the Middle Erian of America, and constitute the most common kind of fossil wood all the way to the Trias. They have in the older formations, generally, several rows of pores on each fibre, and medullary rays composed of two or more series of cells, but become more simple in these respects in the Permian and Triassic series. The names Araucarites and Araucarioxylon are perhaps objectionable, inasmuch as they suppose affinities to Araucaria which may not exist. Unger’s name, which is non-committal, is therefore, I think, to be preferred. In my “Acadian Geology,” and in my “Report on the Geology of Prince Edward Island,” I have given reasons for believing that the foliage of some at least of these trees was that known as Walchia, and that they may have borne nutlets in the manner of Taxine trees (Trigonocarpum, &c). Grand d’Eury has recently suggested that some of them may have belonged to Cordaites, or to plants included in that somewhat varied and probably artificial group.

The earliest discovery of trees of this kind in the Erian of America was that of Matthew and Hartt, who found large trunks, which I afterwards described as Dadoxylon Ouangondianum, in the Erian sandstone of St. John, New Brunswick, hence named by those geologists the “Dadoxylon sandstone.” A little later, similar wood was found by Prof. Hall and Prof. Newberry in the Hamilton group of New York and Ohio, and the allied wood of the genus Ormoxylon was obtained by Prof. Hall in the Portage group of the former State. These woods proved to be specifically distinct from that of St. John, and were named by me D. Halli, D. Newberryi, and Ormoxylon Erianum. The three species of Dadoxylon agreed in having composite medullary rays, and would thus belong to the group Palæoxylon of Brongniart. In the case of Ormoxylon this character could not be very distinctly ascertained, but the medullary rays appeared to be simple.

I am indebted to Prof. J. M. Clarke, of Amherst College, Massachusetts, for some well-preserved specimens of another species from the Genesee shale of Canandaigua, New York. They show small steins or branches, with a cellular pith surrounded with wood of coniferous type, showing two to three rows of slit-formed, bordered pores in hexagonal borders. The medullary sheath consists of pseudo-scalariform and reticulated fibres; but the most remarkable feature of this wood is the structure of the medullary rays, which are very frequent, but short and simple, sometimes having as few as four cells superimposed. This is a character not before observed in coniferous trees of so great age, and allies this Middle Erian form with some Carboniferous woods which have been supposed to belong to Cordaites or Sigillaria. In any case this structure is new, and I have named the species Dadoxylon Clarkii, after its discoverer. The specimens occur, according to Prof. Clarke, in a calcareous layer which is filled with the minute shells of Styliola fissurella of Hall, believed to be a Pteropod; and containing also shells of Goniatites and Gyroceras. The stems found are only a few inches in diameter, but may be branches of larger trees.

It thus appears that we already know five species of Coniferous trees of the genus Dadoxylon in the Middle Erian of America, an interesting confirmation of the facts otherwise known as to the great richness and variety of this ancient flora. The late Prof. Goeppert informed me that he had recognised similar wood in the Devonian of Germany, and there can be no doubt that the fossil wood discovered by Hugh Miller in the Old Red Sandstone of Scotland, and described by Salter and McNab, is of similar character, and probably belongs to the genus Dadoxylon. Thus this type of Coniferous tree seems to have been as well established and differentiated into species in the Middle Devonian as in the succeeding Carboniferous.

I may here refer to the fact that the lower limit of the trees of this group coincides, in America, with the upper limit of those problematical trees which in the previous chapter I have named Protogens (Nematophyton, Celluloxlyon,[BM] Nematoxylon[BN]), though Aporoxylon of Unger extends, in Thuringia, up to the Upper Devonian (Cypridina schists).

[BM] “Journal of the Geological Society,” May, 1881.

[BN] Ibid., vol. xix, 1863.

V.—Scottish Devonian Plants of Hugh Miller and others.
(Edinburgh Geological Society, 1877.)

Previously to the appearance of my descriptions of Devonian plants from North America, Hugh Miller had described forms from the Devonian of Scotland, similar to those for which I proposed the generic name Psilophyton; and I referred to these in this connection in my earliest description of that genus.[BO] He had also recognised what seemed to be plants allied to Lycopods and Conifers. Mr. Peach and Mr. Duncan had made additional discoveries of this kind, and Sir J. Hooker and Mr. Salter had described some of these remains. More recently Messrs. Peach, Carruthers, and McNab have worked in this field, and still later[BP] Messrs. Jack and Etheridge have summed up the facts and have added some that are new.