The Geological History of Plants. Sir John William Dawson

“Journal of the Royal Microscopical Society,” 1885.

      In the “Bulletin of the Chicago Academy of Science,” January, 1884, Dr. Johnson and Mr. Thomas, in their paper on the “Microscopic Organisms of the Boulder Clay of Chicago and Vicinity,” notice Sporangites Huronensis as among these organisms, and have discovered them also in large numbers in the precipitate from Chicago city water-supply. They refer them to the decomposition of the Erian shales, of which boulders filled with these organisms are of frequent occurrence in the Chicago clays. The Sporangites and their accompaniments in the boulder clay are noticed in a paper by Dr. G. M. Dawson, in the “Bulletin of the Chicago Academy,” June, 1885.

      Prof. Clarke has also described, in the “American Journal of Science” for April, 1885, the forms already alluded to, and which he finds to consist of macrospores enclosed in sporocarps. He compares these with my Sporangites Huronensis and Protosalvinia bilobata, but I think it is likely that one of them at least is a distinct species.

      I may add that in the “Geological Magazine” for 1875, Mr. Newton, F. G. S., of the Geological Survey of England, published a description of the Tasmanite and Australian white coal, in which he shows that the organisms in these deposits are similar to my Sporangites Huronensis, and to the macrospores previously described by Prof. Huxley, from the Better-bed coal. Mr. Newton does not seem to have been aware of my previous description of Sporangites, and proposes the name Tasmanites punctatus for the Australian form.

      Here we have the remarkable fact that the waste macrospores, or larger spores of a species of Cryptogamous plant, occur dispersed in countless millions of tons through the shales of the Erian in Canada and the United States.

      No certain clue seemed to be afforded by all these observations as to the precise affinities of these widely distributed bodies; but this was furnished shortly after from an unexpected quarter. In March, 1883, Mr. Orville Derby, of the Geological Survey of Brazil, sent me specimens found in the Erian of that country, which seemed to throw a new light on the whole subject. These I described and pointed out their connection with Sporangites at the meeting of the American Association at Minneapolis, in 1883, and subsequently published my notes respecting them in its proceedings, and in the “Canadian Record of Science.”

Mr. Derby’s specimens contained the curious spiral sea-weed known as Spirophyton, and also minute rounded Sporangites like those obtained in the Erian of Ohio, and of which specimens had been sent to me some years before by the late Prof. Hartt. But they differed in showing the remarkable fact that these rounded bodies are enclosed in considerable numbers in spherical and oval sacs, the walls of which are composed of a tissue of hexagonal cells, and which resemble in every respect the involucres or spore-sacs of the little group of modern acrogens known as Rhizocarps, and living in shallow water. More especially they resemble the sporocarps of the genus Salvinia. This fact opened up an entirely new field of investigation, and I at once proceeded to compare the specimens with the fructification of modern Rhizocarps, and found that substantially these multitudinous spores embedded in the Erie shales may be regarded as perfectly analogous to the larger spores of the modern Salvinia natans of Europe, as may be seen by the representation of them in Fig. 16.