By HENRY ALLEYNE NICHOLSON, M.D., D.Sc., M.A., Ph.D., F.R.S.E., F.G.S., &c. Professor of Natural History and Botany in University College, Toronto. The doctrine of "Colonies," propounded by M. Barrande, has been long before the palæontological world, and is known, at any rate by name, to all students of geology. It is doubtful, however, if there is as clear a comprehension of this subject as its importance would render desirable; and it may, therefore, be of interest to discuss briefly the leading facts upon which this theory is based. In so doing, I shall take the necessary details from M. Barrande's "Défense des Colonies," published in 1870, one of the most valuable of the many palæontological works of this distinguished observer, and I shall confine myself chiefly to a resumé of the facts therein recorded and the deductions drawn therefrom. I. SUB-DIVISIONS OF THE SILURIAN ROCKS OF BOHEMIA. The Silurian Rocks of Bohemia are described by M. Barrande as occupying an elliptical basin, the long axis of which has a N.E., and S.W. direction, and a length of 148 kilometres. The breadth of the basin increases gradually in passing from the N.E. to the S.W., its minimum breadth being about 30 kilometres, and its maximum about 74 kilometres. The Silurians of this basin repose upon granitic and gneissic rocks, and dip inwards. towards a central line. The fossiliferous beds of the entire basin occupy a far from considerable superficial area; and their extent -supposing them not to have been much denuded-would assign to the Silurian sea of Bohemia an area not exceeding 1-60 of the superficies of the Adriatic. The Silurian rocks of the entire basin admit of separation into two primary divisions, an Inferior and a Superior division, corresponding respectively to the Lower and Upper Silurian Rocks of Sir Roderick Murchison. The Inferior Division is composed principally of schists and quartzites; or, as we should say, slates and grits or graywackes, and is wholly destitute of calcareous matter, except occasional concretions of carbonate of lime. The Superior Division is composed almost entirely of calcareous matter, with merely subordinate bands of schists and quartzites. Each division can be satisfactorily broken up into four sub-divisions (étages), grounded solely upon the characters of their contained fossils, and lettered in ascending order : The étages of the Inferior Division are A., B., C., D. The étages of the Superior Division are E., F., G., H. Each of the fossiliferous sub-divisions can be further broken up into minor groups or "bands," distinguished by the smaller letters of the alphabet, as shown in the annexed table. Etages A. & B., the lowest of the Inferior Division, are composed of semi-crystalline rocks and conglomerates, and are unfossiliferous. They are termed by Barrande the "Azoic Etages," and are considered by him as forming the base of the Silurian Series. It is, however, more probable that they should be regarded as being truly of Lower Cambrian age. Etages C. D. E. F. G. & H. are fossiliferous. Etage C. is the well-known "Primordial Zone" of Bohemia, corresponding with the Menevian beds of Britain, and characterized by primordial trilobites of the genera Paradoxides, Olenus, Conocoryphe, Ellip tocephalus, &c. It should probably be regarded as Upper Cambrian. Etage D. contains Barrande's so-called "faune second" or second fauna, and must correspond with the Llandeilo and Caradoc beds of Britain. Etages E. F. G. & H. are characterized by a single fauna termed by Barrande the "faune troisième" or third fauna; and they correspond collectively to the Upper Silurian Rocks of Britain. The precursors (" avanteureurs ") of this "third fauna" in the last portions of the period of the "second fauna" are termed by Barrande the "colonies." They are in the form of bands which are enclosed in the mass of étage D towards its higher part, and which are thus stratigraphically Lower Silurian, but which, nevertheless, contain a predominance of fossils characteristic of the "third fauna," and thus come palaeontologically to belong to the Upper Silurian series. They abound especially in the band d 5, occurring also in d 4, and about twenty of them are known in all. The subjoined table shows in a summary form the general subdivisions and lithology of the rocks of the Bohemian basin, with the principal characteristic fossils: TABLE OF THE SUBDIVISIONS OF THE SILURIAN ROCKS OF BOHEMIA. Different crystalline rocks. d 1. Compact argillaceous schists, rarely metamorphic. Beds of quartzite " (i. e. greywacke), sometimes Black, argillaceous, and micaceous schists. Impure, very micaceous schists of different tints, Colonies, composed of graptolitic schists, with cal- Compact limestone, black or dark gray, not fetid. Nodular limestone, very like g 3, but much thicker. Fissile, argillaceous schists, without quartzites or Fissile argillaceous schists, alternating in thin beds with quartzites, but with no calcareous concretions. Fissile argillaceous schists, without calcareous concretions. Destitute of fossils. These are the "etages azoiques" of Barrande. Probably Lower Cam- "Primordial fauna." comprising 27 trilobites, 5 pteropods, 2 brachiopods, 47 trilobites. 25 cephalopods, 14 pteropods, few gasteropods, brachiopods, 19 trilobites, 1 cephalopod, 8 pteropods; other fossils very rare. 8 trilobites and 9 brachiopods. 26 trilobites, 6 cephalopods, 18 pteropods; rare, gasteropods, brachiopods 4 trilobites, 36 cephalopods; few brachiopods; many graptolites. 15 trilobites, 149 cephalopods, 5 pteropods ; few gasteropods, brachiopods 81 trilobites, 665 cephalopods, 11 pteropods; very many gasteropods, 11 trilobites, 31 cephalopods, 2 pteropods (first Tentaculites); few gas- 4 fishes, 56 trilobites, 55 cephalopods, 10 pteropods; few gasteropods, 6 trilobites, 12 cephalopods, 3 pteropods (Tentaculites common); few bi- 3 trilobites, 86 cephalopods (14 Goniatites), 2 pteropods; few gasteropods, 2 trilobites. 13 cephalopods (3 Goniatites), 3 pteropods (Tentaculites com- II. DISTRIBUTION OF THE COLONIES. The colonial zone occupies a great part of the superficial area and vertical thickness of the band d 5, forming an elliptical zone or belt concentric with the calcareous rocks of the Upper Silurian basin. From this basin the colonial zone is generally separated by schists and quartzites, which form the summit of d 5, and which contain no fossils of an animal nature. On the surface of this zone the colonies are distributed in concentric but discontinuous lines, with irregular intervals between. Each colony is in the form of a lenticular mass, of which the length enormously exceeds the breadth and thickness; and the phenomena of their distribution and their relations to the surrounding rocks prove plainly that they cannot be explained by invoking the agency of mechanical disturbance or faults. Several interbedded traps are found in the colonial zone, regularly interstratified with the colonies, and similar beds are found in band e 1 at the base of Etage E. They all have the form of elongated lenticular masses thinning out at both extremities. As the Silurian rocks of Bohemia form a basin, the colonies are, as a matter of course, found on both sides of the central group of calcareous rocks (Upper Silurian). With the exception of the "Colony Zippe," which is found in d 4, all the colonies are found in the lower portion of d 5; and, like the rocks amongst which they are situated, they dip inwards towards the axis of the basin. III. LITHOLOGY OF THE COLONIES COMPARED WITH THAT OF BANDS e 1, e 2, d 4, & d 5: A. Band, e 2.—This band is the second subdivision of Etage E., and is composed mainly of continuous beds of limestone, often fetid, almost black in colour, and chiefly composed of the debris. of Crinoids. The beds of limestone are separated by thin courses of impure shales containing a few graptolites. Lithologically e 2 differs most markedly both from band e 1 and from the colonies; but nevertheless the palaeontological relationships of the colonial zone are far stronger with e 2 than with e 1, though the mineral characters of e 1 are identical with those of the colonies. B. Band e 1:- -Band e 1 constitutes the stratigraphical base It of Etage E. or of the Upper Silurian Series of Bohemia. consists wholly of Graptolitic Schists, enclosing calcareous spher oids or "anthracolites" and having intercalated beds of trap. Its thickness is very variable, sometimes exceeding 600 metres, and it is always much thicker than band e 2. Lithologically, therefore, as well as in possessing interbedded traps, e 1 differs greatly from e 2. In the same way, the palæontological differences between the two are sufficiently well marked, though they are united by many specific connexions. Each, however, has its own fauna, and the richness of the two is very unequal. Thus, e 1 possesses but 15 Trilobites, whilst e 2 has 81 species; e 1 has yielded no more than 149 Cephalopods, whilst e 2 has yielded the extraordinary number of 665 species; and similar differences are found in the Gasteropods, Bivalves, and Brachiopods. Still, the propriety of retaining e 1 and e 2 on the same stratigraphical horizon is shown by numerous paleontological relationships, amongst which may be mentioned the fact that 68 Cephalopods are common to the two divisions. C. Band d 5-Band d 5 underlies band e 1, and forms the summit of Etage D., or the highest division of the Lower Silurian Series of Bohemia. Its upper portion has a thickness of 100 metres and is composed of alternating thin beds of gray schist and quartzite (graywacke). It is remarkable in being wholly destitute of fossils of an animal nature, having yielded nothing more than a few "Fucoids". This thick deposit, therefore, corresponds with a prolonged and total intermission of the Silurian fauna of the Bohemia area. The thickness of this unfossiliferous formation might serve as an approximate measure of the time which elapsed between the last appearance of the colonial fauna and the definitive appearance of the "third fauna" (Upper Silurian fauna). In certain localities, however, this unfossiliferous mass appears to have undergone partial denudation, prior to the deposition of e 1. the It may be remarked here that the above observation of M. Barrande would seem to indicate a want of conformity between Etage D. and Etage E., such as is found in many other countries between the Lower and Upper Silurian rocks. If this be So, interval between the colonial fauna and the introduction of the third fauna may have been indefinitely long, and cannot even be approximately measured by the thickness of the upper part of d 5. Below this unfossiliferous series, band d 5 is composed of masses of argillaceous schist of different tints, sometimes with subordi |