It would be very difficult to produce satisfactory evidence for the algal nature of many of the supposed fossil algae from Cambrian rocks[247]; there has been a special tendency to recognise algal remains in the oldest fossiliferous strata, due in part no doubt to the fallacy that in that period nothing higher than Thallophytes is likely to have existed. The so-called Phycodes referred to by Credner[248] as characteristic of the Cambrian rocks of the Fichtelgebirge (“Phycoden-Schiefer”) is probably of inorganic origin, and comparable to the genus Vexillum of Saporta[249] and other writers, which Solms-Laubach has described as being formed every day in the soft mud of our ponds where local currents are checked by branches and other obstacles[250]. There are several good specimens of Phycodes in the Bergakademie of Berlin and in the Leipzig Museum which, I believe, clearly demonstrate the absence of all satisfactory evidence of an algal origin.
We may next pass to a short description of a few representative types of algae, which may reasonably be classed under definite families, and accepted as evidence possessing some botanical value.
A. DIATOMACEAE (Bacillariaceae).
This family occupies a somewhat isolated position among the algae, and is best considered as a distinct subdivision rather than as a family of the Phaeophyceae or Brown algae, with which it possesses as a common characteristic a brown-colouring matter.
Single-celled plants consisting of a simple protoplasmic body containing a nucleus and brown colouring matter (diatomin) associated with the chlorophyll. The cell-wall is in the form of two halves, known as valves, which fit into one another like the two portions of a pill-box. The cell-wall contains a large amount of silica, and the siliceous cases of the diatoms are commonly spoken of as the valves of the individual, or the frustules. Diatoms exhibit a characteristic creeping movement, and are reproduced by division, also by the development of spores in various forms[251].
The recent members of the family have an exceedingly wide distribution, occurring both in freshwater and in the sea. Owing to the lightness of the frustules, they are frequently carried along in the air, and atmospheric dust falling on ships at sea has been found to contain large numbers of diatoms[252]. The siliceous valves are abundant in guano deposits, and they have been found also in association with volcanic material. Diatomaceous deposits are now being formed in the Yellowstone Park district; “they cover many square miles in the vicinity of active or extinct hot spring vents of the park, and are often three feet, four feet, and sometimes five to six feet thick[253].” The gradual accumulation of the siliceous tests on the floor of a fresh-water lake results in the formation of a sediment consisting in part of pure silica. Such deposits, often spoken of as kieselguhr or diatomite, and used as a polishing material, occur in many parts of Britain, marking the sites of dried-up pools or lakes. At the northern end of the island of Skye there occurs an unusually pure deposit of diatomite overlain by peat and turf, and extending over an area of fifty-eight square miles. Many of the individuals in this deposit were in all probability carried into the lake by running water, while others lived in the lake and after death their tests contributed to the siliceous deposit[254]. The late Dr Ehrenberg published numerous papers on diatomaceous deposits in different parts of the world, and in his great work, Zur Mikrogeologie[255], he gave numerous and beautifully executed illustrations of such siliceous accumulations. In many of the samples he figures one sees fragments of plant tissues, spores of conifers and ferns, associated with the diatom tests. The occurrence of the pollen grains of coniferous trees in lacustrine and marine deposits is not surprising in view of their abundance in Lake Constance and other lakes. It is stated that the pollen of conifers in the Norwegian fiords plays an important part in the nourishment of the Rhizopod Saccamina[256].
DIATOMACEOUS OOZE.
In the waters of the ocean diatoms are of frequent occurrence, and very widely distributed. Sir Joseph Hooker records the existence of masses of diatomaceous ooze over a wide area in Antarctic regions[257]. Along the shores of the Victoria Barrier, a perpendicular wall of ice, between one and two hundred feet above sea-level, the soundings were found to be invariably charged with diatom remains, and from the base of the ice-wall there appeared to be in process of formation a bank of these tests stretching north for a distance of 200 miles. The more extended researches conducted during the cruise of the Challenger have clearly proved the enormous accumulations of diatoms now being formed on the ocean-bed[258]. South of latitude 45° S. there is now being built up a vast deposit which may be eventually upraised as a fairly pure siliceous rock. From extreme northern latitudes Nansen has recently recorded the occurrence of these lowly organised plants. He writes,—“I found a whole world of diatoms and other microscopical organisms, both vegetable and animal, living in the fresh-water pools on the Polar drift-ice, and constantly travelling from Siberia to the east coast of Greenland[259].” In warmer latitudes diatoms abound in the surface waters, but there they are associated with numerous other forms of the Plankton vegetation. The waters of the Amazon carry with them into the sea large numbers of fresh-water forms, which are floated out to sea and finally added to the rock-building material which is constantly accumulating on the ocean floor[260]. No definite results have so far been obtained as to the geographical and bathymetrical distribution of marine diatoms.
The enormous number of recent species precludes any attempt to give a description of the better-known forms. It is more important for us to realize how common and widely distributed are the living genera. The hard and almost indestructible valves have been frequently found in a fossil condition, often forming thick and extensive masses of siliceous rock. From diatom-beds now forming in lakes and on the ocean-bed we pass to deposits such as those in Skye and elsewhere, which mark the site of recently dried-up sheets of water, and so to older rocks of Tertiary age formed under similar conditions. Among the many examples of diatomaceous deposits of Tertiary and Cretaceous age mention should be made of those of Berlin, Königsberg, Bilin in Bohemia, and Richmond in Virginia. The diatoms in the beds of Berlin are regarded as fresh-water, and those of Richmond as marine. It has been pointed out by Pfitzer that it is a comparatively easy matter to distinguish between fresh-water and marine forms of diatoms. The diatomaceous rocks of Bilin are known as polishing slates; they attain a thickness of 50 feet. In these, as in many other cases, the deposit has become cemented together as a hard flinty or glassy rock, in which the cementing material was formed by the solution of some of the diatom tests[261]. In many cases in which calcareous and siliceous rocks reveal no direct evidence of organic origin it is probable that they were originally formed by the accumulation of plants of which the structure has been completely obliterated by secondary causes. The genus Gallionella plays an important part in the composition of the Bilin beds. Occasionally impressions of leaves and other organic remains are found associated with the diatoms in the siliceous rocks. In the British Museum (Botanical department) a large block of white powdery rock is exhibited as an example of a diatomaceous deposit of Tertiary age from Australia. It is described as being largely made up of the tests of fresh-water diatoms, such as Navicula, Gomphonema, Cymbella, Synedra, and others.
FOSSIL DIATOMS.