fauna is unfounded, geographical provinces having existed in the oldest and in the most modern times.

Colonies.-M. Barrande, a very distinguished palæontologist, noticed, many years ago, that groups of fossils belonging normally to higher and later strata were intercalated amongst strata containing older forms of living things. He called these precursory collections Colonies,' and defined the phenomena as consisting in the partial co-existence of two faunas, which were nevertheless successive. Thus he supposed that, during the later stages of his second Silurian fauna in Bohemia, the first phases of the third fauna had already appeared, and attained some degree of development in some neighbouring but yet unknown region. He supposed that at intervals which corresponded with changes in the physical geography of a considerable area due to upheaval or subsidence, &c., communication was opened between that outer region and Bohemia. During these intervals, a greater or less number of immigrants succeeded in making their way into the Bohemian area, but as the conditions for their lasting there were not yet favourable, they soon died out, and the normal fauna of the region spread over the invaded area, but on a higher horizon. After other changes had taken place and the prevailing fauna had diminished or died off, an invasion from the distant region was followed by the full development of the parent stock (or rather of its descendants), parts of which had long before migrated without results. A remarkable instance of a so-called 'Colony' was given in explaining the affinities of the lowest Tertiaries of Belgium. Barrande's explanation may be accepted in the case of the Belgian strata ; but geologists have hesitated to believe the theory in other instances, and have attributed the phenomena to the inversion of strata. But M. Barrande insists that there are no such inversions in the typical region which he studied, and indeed, from what we now know about the existence of differing neighbouring faunas being only separated by narrow tracts of land, the theory is not unreasonable. Nevertheless, the examples are rare, for in the vast majority of instances the normal succession of characteristic fossils prevails. The instance of the existence of Serpula and Conularia, Carboniferous forms, in the upper Old Red of South Wales and Shropshire is that of a Colony; but that finding of Silurian marine fossils in the Old Red (already noticed, p. 419), would not be a discovery of a Colony in Barrande's sense.

CHAPTER XXVIII.

CAMBRIAN, PRE-CAMBRIAN,. AND LAURENTIAN FORMATIONS. Classification of the Cambrian group-Upper Cambrian rocks-Tremadoc slates and their fossils-Lingula flags-Lower Cambrian rocks-Menevian beds-Longmynd group-Harlech grits, with large TrilobitesLlanberis slates-The succession in the North-west Highlands-Cambrian rocks of Bohemia-Primordial zone of Barrande-Metamorphosis of Trilobites-Cambrian rocks of Sweden and Norway-Cambrian rocks of the United States and Canada-Potsdam sandstone-Pre-Cambrian series-English and Welsh Pre-Cambrians-Canadian, Huronian seriesLaurentian group, upper and lower-Eozoon Canadense, oldest known fossil-Fundamental gneiss of Scotland-Other gneissic areas-Archæan and Azoic series.

CAMBRIAN FORMATION.

THE Arenig group is separated from the underlying and conformable Tremadoc strata by a fairly marked palæontological break, and the strata of this last group are placed at the top of the Cambrian formation. The following is the succession of the Cambrian strata :

UPPER CAMBRIAN.

TREMADOC SLATES. (Primordial of Barrande in part.)

LINGULA FLAGS.

(Primordial of Barrande.)

LOWER CAMBRIAN.

MENEVIAN BEDS. (Primordial of Barrande.)

LONGMYND GROUP b. Llanberis Slates.
a. Harlech Grits.

UPPER CAMBRIAN.

Tremadoc Slates.-The Tremadoc slates of Sedgwick are more than 1,000 feet in thickness, and consist of dark earthy grey slates occurring near the little town of Tremadoc, situated on the north side of Cardigan Bay in Caernarvonshire.

They were traced subsequently to Dolgelly, and of late years strata of the same age have been discovered and carefully examined by Dr. Hicks, at St. David's promontory and Ramsey Island, South Wales, where there are dark earthy flags and sandstones 1,000 feet thick, with many fossils. They rest conformably upon thick Lingula flags. Subsequently Mr.

Callaway has shown that the Shineton shale of Shropshire is of Lower Tremadoc age. The fauna is very remarkable, and differs in North and South Wales considerably, numbering at least 84 species, and many great groups of the invertebrata appear in the rocks for the first time. The Crinoidea, Asteroidea, Cephalopoda, and Lamellibranchiata are represented therein, for the first time in the world's history. There are many new genera of Trilobita, such as Nesuretus, Psilocephalus, Niobe, Angelina, Asaphus, and Cheirurus, besides some which existed in the lower rocks, such as Agnostus, Conocoryphe, and Olenus. The Crinoid Dendrocrinus and Asteroid Palæasterina, the Cephalopoda Orthoceras sericeum and Cyrtoceras præcox are of the Upper Tremadocs, and

A

Fig. 572.

Fig. 573.

Cyrtoceras precox, Salt, mag.
Llandeilo and Tremadoc rocks, N. Wales.

a. Dorsal edge, place of siphuncle.
b. Aperture. c. Ventral edge.

Theca (Cleidotheca)
operculata, nat. size.
Lower Tremadoc beds,
Tremadoc.

the Lamellibranchs Ctenodonta, Palæarca, Glyptarca, Davidia, and Modiolopsis are the first known. The Brachiopoda belong to the genera which existed in the underlying strata, and the species Lingulella Davisii and Orthis Carausii, and the genera Obolella and Lingula, are common to both groups. The North Wales Tremadocs contain 9 species of Pteropoda, of the genus Theca principally; and Bellerophon is found amongst the Heteropoda. The earliest Rhabdophora were discovered, in Tremadoc rocks, by Callaway, and belong to the genus Bryograptus. Phyllopod Crustacea exist in the Upper Tremadocs, and the characteristic Trilobita are Angelina Sedgwickii, Conocoryphe olenoides, and Olenus impar. Dictyonema sociale and Bryozoa occur, and in the strata below also.

Lingula Flags.-Next below the Tremadoc slates in North Wales, lie micaceous flagstones, bluish and black slates and flags, with bands of grey flags and sandstones, in which in 1846 Mr.

E. Davis discovered the Lingulella (fig. 575) named after him, and from which was derived the name of Lingula flags. These beds are more than 5,000 feet thick, and have been studied chiefly in the neighbourhood of Dolgelly, Ffestiniog, and Portmadoc in North Wales, and also at St. David's in South Wales. They have yielded 26 genera and 69 species of fossils, of which 9 only are common to the overlying Tremadoc rocks. They are Dictyonema sociale, Agnostus princeps, Ampyx prænuntius, Conocoryphe depressa, Olenus impar (?), Lingulella Davisii, L.

'Lingula Flags' of Dolgelly, and Ffestiniog, N. Wales.

lepis, Obolella, and Orthis. In the Lingula flags Olenus (fig. 576), Agnostus, Anopolenus, Microdiscus, Paradoxides, and Conocoryphe are prominent forms of Trilobita, and Hymenocaris vermicauda (fig. 574), is a common species of Phyllopod Crustacea.

The Lingula Flags may be divided into two zones, an upper and lower, the old middle zone being of no value, and amongst the fossils of the upper zone is Dictyonema sociale, which occurs in the dark shales of Keys End Hill, Malvern, and in North Wales. Two genera of Annelida are also found in the Holybush Sandstone of Malvern, at the base of the Upper Lingula Flags. No less than 30 species of Crustacea belonging to the genera of Trilobita, just noticed, occur, and only 4 pass up into the Tremadocs. The Brachiopoda are of 8 species, 6 of which pass upwards, and the genera are Lingula, Lingulella, Obolella, Kutorgina, and Orthis, the two characteristic species being Lingula pygmæa and Obolella Salteri. In the Lower Lingula Flags, which rest conformably on the Menevian strata, Cruziana, a supposed Annelid, occurs, and Scolioderma and Helminthites are characteristic worms. Nine genera and 25 species of Crustacea are found. Agnostus limbatus and A. nodosus, Olenus cataractes, O. micrurus, O. gibbosus, are peculiar to these lower Flags, and so is the Phyllopod Hymenocaris vermicauda. The three genera of Brachiopoda are Lingulella, Orthis,

GG

and Obolella. Finally, two species of Theca occur. It will be observed how many great groups of invertebrata are not represented, they not having yet appeared.

In Merionethshire, according to Sir A. Ramsay, the Lingula Flags attain their greatest development; in Caernarvonshire, they thin out so as to have lost two-thirds of their thickness in eleven miles; while in Anglesea and on the Menai Straits, both they and the Tremadoc beds are entirely absent, and the Lower Silurian rocks rest directly on Lower Cambrian strata.

LOWER CAMBRIAN.

Menevian Beds.-Immediately beneath the Lingula Flags there occurs a series of dark-grey and black flags and slates, alternating at the upper part with some beds of sandstone, the whole reaching a thickness of from 500 to 600 feet. These beds were formerly classed, on purely lithological grounds, as the base of the Lingula Flags; but Messrs. Hicks, Salter, and Etheridge,

Fig. 577.

to whose exertions we owe almost all our knowledge of them, have pointed out that the most characteristic genera found in them are unknown in the Lingula Flags, while they possess many forms from the underlying Longmynd Group. They therefore proposed to place these beds, and it seems to me with reason, at the top of the Lower Cambrian under the term 'Menevian,' Menevia being the classical name of St. David's. The beds are well exhibited in the neighbourhood of St. David's in South Wales, and near Dolgelly and Maentwrog in North Wales. They are the equivalents of Étage C of Barrande's Primordial Zone. Fifty-two species have been found in the Menevians, which are very rich in fossils for so early a period. Nineteen species are common to the overlying Lingula Flags, but none pass up to the Tremadoc beds. Twelve genera and 32 species of Trilobita occur, and some forms are of large size; Paradoxides Davidis (see fig. 577), the largest Trilobite known in Great Britain, 22 inches or nearly 2 feet long, is peculiar to the Menevian, and the gigantic P. aurora also. The other genera are Agnostus, Anopolenus, Conocoryphe, Holocepha