Peilophyton princeps. Dawson, Geol. Quart. Journ., vol. xv., 1863; and Canada Survey, 1863. Species characteristic of the whole Devonian period in North America. A. Psilophyton princeps, plant restored by Dawson. Ac. Cylindrical rootlets. d. Rhizome. e. Areole of rhizome. f. Stem, twice natural size. g. Termination of branches. gen; but with this single exception the American Devonian flora affords, like the Carboniferous, no evidence of the existence of plants of higher organization than the gymnosperms. The monotonous character of the Carboniferous flora might be explained by imagining that we have only the vegetation handed down to us of one set of stations, consisting of wide swampy flats. But Dr. Dawson supposes that the geographical conditions under which the Devonian plants grew were more varied, and had more of an upland character. If so, the limitation of a flora represented by so many genera and species to the gymnospermous and cryptogamous orders, and the absence of plants of higher grade, admit of no expla nation hitherto advanced save that afforded by the theory of progressive development. Nothing is known of the insects, land-shells, or other terrestrial animals which coexisted with this Devonian flora, but we need not despair of future discoveries in this direction when we remember that slow as has been our progress, we have at length begun to learn something respecting the terrestrial fauna of the Coal period. Allusion has already been made to freshwater shells and to Lepidodendra and ferns (see figs. 585 and 586, p. 524) found in Ireland associated with Devonian genera of fish. CHAPTER XXVII. SILURIAN AND CAMBRIAN GROUPS. Silurian strata formerly called Transition-Term "Grauwacke"-Subdivisions of Upper, Middle, and Lower Silurians-Ludlow formation and fossils-Oldest known remains of fossil fish-Wenlock formation, corals, cystideans, trilobites— Middle Silurian or Llandovery Beds-Lower Silurian rocks-Caradoc and Bala Beds-Upper and Lower Llandeilo formations-Cystidea-Trilobites-Graptolites-Vast thickness of Lower Silurian strata, sedimentary and volcanic, in Wales-Foreign Silurian equivalents in Europe-Silurian strata of the United States-Amount of specific agreement of fossils with those of Europe-Canadian equivalents-Whether Silurian strata of deep-sea origin-Cambrian rocks— Classification and nomenclature-Barrande's primordial fauna -- Upper Cambrian of Wales-Tremadoc slates-Lingula flags-Lower Cambrian-Longmynd group-Oldest organic remains known in Europe-Foreign equivalents of the Cambrian group-Primordial zone of Bohemia-Characteristic trilobitesMetamorphosis of trilobites-Alum schists of Sweden and Norway-Potsdam sandstone of United States and Canada-Footprints near Montreal Quebec strata and Huronian rocks-Minnesota trilobites-Rocks older than the Cambrian-Laurentian group, Upper and Lower-Oldest known fossil, Eozoon Canadense-No remains of vertebrate animals known in strata below the Upper Silurian-Progressive discovery of vertebrata in older rocks-Theoretical inferences from the rarity or absence of vertebrata in the most ancient fossiliferous formations. WE come next in the descending order to the most ancient of the primary fossiliferous rocks, that series which comprises the greater part of the strata formerly called "Transition" by Werner, for reasons explained in Chapter VIII., p. 89. Geologists were also in the habit of applying to these older strata the general name of "Grauwacke," by which the German miners designate a particular variety of sandstone, usually an aggregate of small fragments of quartz, flinty slate (or Lydian stone), and clay-slate cemented to gether by argillaceous matter. Far too much importance has been attached to this kind of rock, as if it belonged to a certain epoch in the earth's history, whereas a similar sandstone or grit is found in the Old Red, and in the Millstone Grit of the Coal, and sometimes in certain Cretaceous and even Eocene formations in the Alps. The annexed table will explain to the reader the successive formations into which the strata called Silurian by Sir Roderick Murchison may be subdivided: The name of Silurian was first proposed by Sir Roderick Murchison for that great series of fossiliferous strata which lie immediately below the Old Red Sandstone, and occupy that part of Wales and some contiguous counties of England which once constituted the kingdom of the Silures, a tribe of ancient Britons. This member of the Upper Silurian group, as will be seen by the above table, is about 800 feet thick, and subdivided into two parts— the Upper Ludlow and the Lower Ludlow-at or near the top of which last occurs the Aymestry limestone. Each of these may be distinguished near the town of Ludlow, and at other places in Shropshire and Herefordshire, by peculiar organic remains. Upper Ludlow.-a. Downton Sandstone.-This uppermost subdivision was originally classed by Sir R. Murchison, under the name of "Tilestones," with the Old Red Sandstone, the beds being often of a similar red color. The whole was regarded as a transition group forming a passage from the Silurian strata to Old Red Sandstone; but it is now ascertained that the fossils agree in great part specifically, and in general character entirely, with those of the underlying Upper Ludlow rocks. Among these are Orthoceras bullatum, Platyschisma helicites, Bellerophon trilobatus, Chonetes lata, &c., with numerous defences of fishes. These beds are well seen at Kington in Herefordshire, and at Downton Castle near Ludlow, where they are quarried for building. Bone-bed. The bone-bed of the Upper Ludlow deserves especial notice as affording the most ancient example of fossil fish occurring in any considerable quantity. It usually consists of one or two thin brown layers full of bony fragments near the junction of the Old Red Sandstone and the Ludlow rocks, and was first observed by Sir R. Murchison near the town of Ludlow, where it is three or four inches thick. It has since been traced to a distance of 45 miles from that point into Gloucestershire and other counties, and is commonly not more than an inch thick, but varies to nearly a foot. At May-Hill two bone-beds are observable, with 14 feet of intervening strata full of Upper Ludlow fossils.* At that point immediately above the upper fish-bed numerous small globular bodies have been found, which were determined by Dr. Hooker to be the sporangia of a cryptogamic land-plant, probably lycopodiaceous. These beds occur just beneath the lowest strata of the "Old Red,” forming the uppermost part of the Downton sandstone. Most of the fish have been referred by Agassiz to his placoid order, some of them to the genus Onchus, to which the spine (fig. 619) and the minute scales (fig. 620) are supposed to belong. It has been sug Fig. 619. Fig. 620. Onchus tenuistriatus, Agass. Shagreen scales of a placoid fish gested, however, that Onchus may be one of those Acanthodian fish, referred by Agassiz to his Ganoid order, which are so characteristic Fig. 621. of the base of the Old Red Sandstone in Forfarshire, although the species of the Old Red are all different from those of the Silurian beds now under consideration.† The jaw and teeth of another predaceous genus (fig. 621) have also been detected, together with some specimens of Pteraspis Ludensis. As usual in bone-beds, the teeth and bones are, for the most part, fragmentary and rolled. Plectrodus mirabilis, Agass. b. Gray Sandstone and Mudstone, &c.-The next subdivision of the Upper Ludlow consists of gray calcareous sandstone, or very com * Murchison's Siluria, pp. 137-237. Powrie, Geol. Quart. Journ., vol. xx. p. 438. |