*a/The Lower Silurian ། དབཱ ༠ མ པ

invertebrali? pined; for rondhus/species not yn decided not the former - very like that ९ uf Llandeilo flags/ uus lower Silurian rocks of Bala.

GEOLOGY,

84 of the Onchus Murchison of the Ludlow Rock, that i-clear the Silurian system great naluval History Series, as - provid lowing detailed section will convey to the student a 'more correct idea of the order and succession among these systems :1. Slightly micaceous thin-bedded sandstones.

SILURIAN.

GRAU-
WACKE.

CLAY-SLATE.

2. Gray and blue argillaceous limestones.

3. Liver-coloured shale, with concretions of earthy limestone.

4. Highly concretionary subcrystalline blue and gray limestone.

5. Dark-gray argillaceous shale, with nodules of earthy limestone.

6. Thin-bedded impure limestone, containing shells, alternating with finely-laminated micaceous sandstone of a greenish colour.

7. Sandstones, grits, and limestones; arenaceous beds prevailing.

8. Dark-coloured flags, chiefly calcareous; sandstones and sandy schists.

9. Grauwacke slates and sandstones, coarse limestones, and thick-bedded grauwacke rocks. 10. Dark argillaceous limestone, with shells and corals. 11. Peculiar slaty and flaggy beds: mottled in colour; sometimes coarse and conglomerate, generally of moderate fineness, alternating with coloured clayey beds.

12. An immense thickness of clay-slate of various colours--blue, black, greenish, purple, or mottled; of fine grain, sometimes compact, sometimes soft and useless.

160. The organic remains of these systems are possessed of more than ordinary interest, from the fact of their being the earliest forms of life with which we are acquainted. In the preceding systems we have no traces either of vegetable or of animal existence life begins to dawn only with the development of the clay-slate group, and to become more abundant as the deposition of the grauwacke and silurian proceeds. The earliest forms of vitality are not plants, but animals-animals undoubtedly low in the scale of organised being, but still perfect animals, as perfectly adapted to the condition of things under which they had to live as those now existing. They are wholly marine; and here it may be observed, that no remains of terrestrial animals have yet been discovered by geologists earlier than towards the close of the Secondary Formations. In the clay-slate and grauwacke, no traces of vegetable organism have been found, and only about thirty species of corals and shell-fish; in the silurian, animal remains become much more abundant; but doubt is entertained respecting some fragments of sea-weeds and ferns said to belong to this formation Whether the remains of plants and animals were entombed in these earlier formations, and have since been * See bolow p. 119

inded 16 other organie remains" (Murchison to Millar) of date 1847 The Bala Spine'n "detected ~ Leakcoram matrix by Leologisu Sournment Survey/ 41 Sir Rodenck, as that Onchuse.

85

CLAY-SLATE, GRAUWACKE, AND SILURIAN SYSTEMS, competent authority in such matters, Professor Edward Forbes. obliterated by the agency of heat, geological science has not been able to determine; but at present, we are warranted in stating, that only a few rare corals and shells occur in the clayslate system; a greater number of corals, shells, and crustacea in the grauwacke; and a variety of corals, shells, crustacea, fish bones, and teeth, in the silurian.So far as they have been examined, all these remains belong to species long since extinct; indeed, are distinct from those which occur in the Secondary Strata, and bear only a generic resemblance (often a faint one) to existing races.

161. From the scantiness and peculiarity of organic life, it is difficult to arrive at any conclusion as to the condition of the world at this early period. The existence of shell-fish would seem to indicate the co-existence of marine plants upon which they fed; and though we are aware that molluscous animals might prey upon each other, still, the probability is, that marine vegetation was to some extent spread over the bottom of the transition seas. The preponderance of corallike animals points to a warm and favourable condition of the waters for their development; and it may be that this highly heated condition entirely prevented the growth of terrestrial plants, and rendered even those of marine growth of so rare Occurrence. Be this as it may, we are only certain of several species of Zoophytes, Mollusca, and Crustacea in the clay-slate and grauwacke rocks-among which the most characteristic are those exhibited in the following group :

1. Cyathophyllum Cyathus; 2. Heliopora Porosa; 3. Catenipora Labyrinthica ; 4. Producta; 5. Spirifera; 6. Terebratula.

The anauntiation as cristones spines Onchus in Flandrilo Flags nowe I Professor Sedgwick. Below Islandula flags, lality deemed & hast Iron Silurian formation fassiliferous bed Mihickness bubong 1 great natural history someo - Trpill Sin Red. Murchison - From Witness, Jon! 27. 1849..

In Onondago linestone/a formation to lawer & more from bottom) defensive spiner 2 inches: But the defensive spine ronchus M. the or, mutilated at bott Ludlow rocks/a defensive spine

end 3 inches 5/8 porto in leng" has been found, which the comparatively more a more frame than the Upper

spines Sonchus (for it measures

only 3 inches in lixoth) is at least 5 times more bulky than

the

larger

Than the

spine of that

ལས་་་་་་་ OTSU 4 ་་་

ald Red Sandstone: The oldest order (Placord) of class
fishon is order of up by scene Things is or of largely
86 represented/ of the primitive orders bony coverings are
net (or & Ganoids) contiOLOGY that interrupted &intcoular
and the greater part of their internal skeleton, not osseous
162. The fossils of the silurian system are much more abun-
'dant; there are a greater number of species belonging to each
genus; annulosa, crustacea, and fishes, are decided; and, ac-
cording to many geologists, plants allied to the alga (sea-
weeds), the equisetacea (horse-tails), filices (ferns), and others,
make their appearance. Passing over the remains of plants
and true vertebrated fishes, about the existence of which at
this period there is still great obscurity, we shall notice some
of the ascertained peculiarities of the radiata, mollusca, and
articulata belonging to the Transition era. /Senatural and en
163. The waters of the silurian period seem to have been
crowded in some localities with zoophytes and corals, for cer-
tain limestones are as much composed of their remains as a
coral reef is of recent corals. Among the radiata, the crinoid
or encrinite family occur for the first time, these differing
from other corals (see par. 300) in the self-dependent nature
of their structure, their fixed articulated stalk, and floating
stomach furnished with movable rays for the seizure and
retention of their food. The shell-fish also become more nu-
merous and distinct in form; spiriferæ, terebratulæ, and pro-
ducta, are every where abundant: and chambered shells, like
the existing nautilus, begin to people the waters. It must
be remarked, however, that the encrinites and chambered
shells of this early period are not so numerous, so gigantic, or
so perfect in their forms, as those of the Secondary Strata; it
is in the mountain limestone group that the crinoidea attain
their meridian, and in the lias and oolite that the ammonites
and nautili are most fully developed. Of the crustacea of
this era, the most interesting and abundant type is the trilo-
bite (three-lobed), of which several genera and many species
have been described, and to which scarcely any existing crea-
ture bears an analogy. The trilobite (see 7, 8, 9 in the fol-
lowing engraving) was a true crustacean, covered with shelly
plates, terminating variously behind in a flexible extremity,
and furnished with a head-piece composed of larger plates, and
fitted with eyes of a very complicated structure. It is sup-
posed by some to have made its way through the water by
means of soft paddles, which have not been preserved; and by
others merely to have sculled itself forward by the aid of its
flexible extremity. Of its various organs, the most interesting
is the eye, of which several specimens have been obtained in
a very perfect state. This organ, according to fossil anato-
mists, is formed of 400 spherical lenses in separate compart-
ments, on the surface of a cornea projecting conically upwards,
so that the animal, in its usual place at the bottom of waters,
could see everything around. As there are two eyes, one of
but cartilaginous wa nchi your destructive park
such as dermal plates or prickler defensive spilles & Teath
Aganrir days & living generaln's Onchus the must be

Compared are the genita Cestracion/Centrina/& Spinal. (p.
Spreimen、 Spinay Ac. 2 feet 3 indies posterior defensive

Susidea)

CLAY-SLATE, GRAUWACKE, AND SILURIAN SYSTEMS.

87

inner

the sides of each would have been useless, as it could only
look across to meet the vision of the other; but on the
sides there are no lenses, that nothing may, in accordance
with a principle observable throughout nature, be
away. It is found that in the serolis, a surviving
genus, the eyes are constructed on exactly the same principle,
except that they are not so high-a necessary difference, as

thrown kindred

6

3

Fram

positive least same locality &

throughout

bulk

fully rivals the Onondage one. From Wilness, Jan. 29.1849) a nearly equal thickness (indicating very considerable size) and which in found a spine still fragmentary; for it length in only 23/8 inches, which maintains side) larger spine Spinay A. and lower stills Oriskany Sandstone

9a

trivial

1. Astrea; 2. Turbinolia Fungites; 3. Terebratula Risca; 4. Leptæna Lata; 5. Actinocrinites; 6. Euomphalus Rugosus; 7. Asaphus de Buchii; 8. Asaphus Tuberculatus; 9. Calymene Blumenbachii; 9a. Side view of Calymene while rolled up. the back of the serolis is lower, and presents less obstruction to the creature's vision. This little organ of little animal carries to living man the certain knowledge that, many ages ago, the air he breathes, and the light by which he sees, were the same as at this hour, and that the sea must have been in general as pure as it is now. If the water had been constantly turbid or chaotic, a creature des"The great number Trilobite feature older & Carboni -Serous & especially Silurian rocks. No country richer usos United States, & m2 Conrad 92: table of distinct sentra Trilobites which characterize, his Upper, Middle Tower (turn over)

་

Hower formations New York. (Byell's america, Vol I 53) The lowest rock u a Traces of fishes have found is in

Angrica; viz. in the bonam upper av top is lower Silurian

88

GEOLOGY

Series, Traced & weat Virginia North Branch

66

tined to live at the bottom of the sea would have had no use for such delicate visual organs. "With regard to the atmosphere," says Dr Buckland, we infer that, had it differed materially from its actual condition, it might have so far affected the rays of light, that a corresponding difference from the eyes of existing crustaceans would have been found in the organs on which the impressions of such rays were then received. Regarding light itself also, we learn, from the resemblance of these most ancient organisations to existing eyes, that the mutual relations of light to the eye, and of the eye to light, were the same at the time when crustaceans endowed with the faculty of vision were placed at the bottom of the primeval seas, as at the present moment."

164. The animals of this early period, like those now existing, were partly herbivorous (living on plants), and partly carnivorous (living on the flesh of others). The polypes and crinoidea, it is true, merely secreted limy matter from the waters of the ocean wherewith to build their calcareous structures; but while certain tribes of shell-fish were living on the sea-weeds which flourished along the shores, other races were preying upon these, or upon each other. Among the vegetable eaters were the productæ, terebratulæ, &c.; the ammonites and trilobites were those which lived upon others.

165. That the transition strata have been derived partly from the disintegrated materials of the gneiss and mica schists, and partly from the granite, is abundantly obvious. In the gneiss and mica schists, the primitive crystals of the granite in many instances had undergone little attrition, and in most cases only sufficient to make them arrange themselves in a flat or laminated position. In the transition systems, the material has suffered sometimes both a mechanical and chemical change. The felspar of the granite and primary strata presents itself in the clay-slate as a soft argillaceous sediment, destitute of the potash and soda which entered into its crystallised condition. The quartz presents itself in sandy grains, without any particular form, sometimes finely pulverised, at other times coarse and gritty. The mica is variously disseminated, scarcely appearing in some strata of the grauwacke; but occurring in others of the silurian, so as to give them a micaceous and laminated aspect. All this implies the combined agency of air and water-the atmosphere to assist in the chemical decomposition of the felspar, water to transport it to the sea of deposit. The fineness and thickness of the clay-slate deposit indicates an immense depth of still-water; the sandy and conglomerate beds of the grauwacke not only the action of rivers, but the action of the sea upon its shores; while the calcareous beds of

Susquehanna, Pensylvania. (yell's Americas)
Se for latest discovered fossik in dilurian system,
page 227

Su also Athen. 1851, p. 1350..