Sea-Weeds, Shells and Fossils Part 11

[Ill.u.s.tration: _Orthoceras subannulatum_ (from the Silurian).]

5. Cambrian. Under this term, derived from the old name for Wales, are included many sandstones, grits, slates and flags, with here and there a limestone band. They form the greater part of the western counties of Wales, where they rise to a considerable height above the sea level. The highest hills of Westmoreland and more than half of Scotland are composed of beds of this age.

The fossils, save in the limestone bands, are not easy to find, but in places they are fairly abundant. Brachiopods are far more numerous than the Mollusca properly so-called. Of these, the genus _Orthis_ was most abundant at about the close of this period. Certain beds of this age have received the name of Lingula Flags, owing this prevalence in them of the curious Brachiopod _Lingula_ so like the species now living in some of the warm seas of the tropics. The Trilobites included several forms, and one species (_Paradoxides Davidis_) attained the length of nearly two feet. A few star-fish, some Hydrozoans (_Graptolites_), and the tubes and casts of Annelides and tracks of Trilobites, complete the list of more remarkable fossils. The subdivisions of the Cambrian rocks will be found in the table on p. 16.

6. Pre-Cambrian.--Near St. David's Head and some other places in Wales, in Anglesea, Shropshire, etc., some yet older rocks have been found.

They are probably for the most part of volcanic origin, but they have been so much changed since they were first deposited, and as. .h.i.therto no fossils have been found in them, little is known concerning them.

Parts of the western coast of Northern Scotland and the Hebrides are composed of a crystalline rock called Gneiss, and supposed to be the oldest member of the British strata. No fossils have been found in it.

[Ill.u.s.tration: Skull of _Deinotherium giganteum_, a huge extinct animal, related to the elephants (from the Miocene of Germany).]

VOLCANIC ROCKS. Although there are fortunately no volcanoes to disturb the peace of our country at the present day, there is abundant evidence of their existence in the past. Not only are some of the beds, especially those of Paleozoic age, composed of the dust and ashes thrown out of volcanoes, with here and there a lava flow now hardened into solid rock, but the stumps of the volcanoes themselves are left to tell the tale. The cones indeed are gone, carried off piecemeal by the rain and frosts, and other destructive agencies, in the course of countless ages: not so the once fluid rock within; _that_ cooled down into Granite, and though originally below the surface, it now, owing to the removal of the overlying softer strata, forms raised ground overlooking the surrounding country. The granite ma.s.ses of Cornwall, of Dartmoor, in the south-west of Mt. Sorrel; the variety called Syenite at Malvern and Charnwood Forest; the Basalts of the Cheviot Hills and of Antrim; the volcanic rocks of Arthur's Seat, Edinburgh, and of the islands of Skye and Mull, etc., are examples of this cla.s.s of rock. They are of different ages, and belong to different periods of the earth's history, from early Palaeozoic down to Miocene times.

TABLE OF THE PRINc.i.p.aL DIVISIONS OF THE ANIMAL KINGDOM, TO SHOW THE ORDER IN WHICH THE FOSSILS SHOULD BE ARRANGED.

INVERTEBRATA.

_Foraminifera_, minute chambered sh.e.l.ls like the Nummulite.

_Spongida_, Sponges.

_Hydrozoa_, Graptolites, etc.

_Actinozoa_, Corals.

_Echinodermata_, Sea-urchins, Stone-lilies, Starfish, etc.

_Annelida_, Worm tracks.

_Crustacea_, Trilobites, Crabs, etc.

_Arachnida_, Scorpions and Spiders.

_Myriapoda_, Centipedes.

_Insecta_, Beetles, b.u.t.terflies, etc.

_Polyzoa_ (_Bryozoa_) or Moss Animals.

_Brachiopods_, Lampsh.e.l.ls.

{ _Lamellibranchiata_, Bivalves.

_Mollusca_ { _Gasteropoda_, Univalves.

{ _Cephalopoda_, Cuttlefish, Ammonites.

VERTEBRATA.

_Pisces_, Fish.

_Amphibia_, Labyrinthodonts, Frogs, and Newts.

_Reptilia_, Reptiles.

_Aves_, Birds.

_Mammalia_, Mammals.

WORKS OF REFERENCE.

FOR NAMING COMMON FOSSILS.

=Tabular View of Characteristic British Fossils Stratigraphically Arranged.= By J. W. LOWRY. _Soc. Prom. Christ. Knowledge._ 1853.

=Figures of the Characteristic British Tertiary Fossils (Chiefly Mollusca) Stratigraphically Arranged.= By J. W. LOWRY and others. _London_ (_Stanford_). 1866.

PALaeONTOLOGY.

=The Ancient Life History of the Earth.= By H. A. NICHOLSON. 8vo. _Edinburgh and London._ 1877.

=A Manual of Palaeontology.= By H. A. NICHOLSON. 2nd edition. 2 vols. 8vo. _Edinburgh and London._ 1879.

PETROLOGY.

=The Study of Rocks.= By F. RUTLEY. (Text Books of Science.) 8vo. _London._ 1879.

FIELD GEOLOGY.

=A Text-Book of Field Geology.= By W. H. PENNING. With a Section on Palaeontology, by A. J.

JUKES-BROWN. 2nd edition. 8vo. _London._ 1879.

GEOLOGY IN GENERAL.