types do not appear freely-firstly, because the composition of such rocks, whether they are sedimentary or igneous in origin, is not suited to the development of large felspars and coarse structures under metamorphic action combined with movement; secondly, because any such bolder crystals already existing in a primary basic mass become, from their composition, readily broken down; these recrystallise in granular and microlitic forms, so that the ultimate result of earth-pressure is a finegrained rock which one would class without hesitation with the schists. To take an extreme case, serpentine-schists abound; but an olivine- or a serpentine-gneiss would seem a structural impossibility.

On the other hand, it is very possible that highly silicated rocks are really more abundant in the outer layers of the earth's crust, giving rise to large areas of gneiss. The occurrence of basic rocks would then be more of the nature of an accident, and would be due to the local protrusion of matter from a lower level, or to the refusion of material already so protruded.

Note.-GRANULITE is a term for a great group of rocks, mostly metamorphic, which have as a common character a fine-grained granular structure. They are of the most varied composition, and must be regarded as structural varieties of a number of well-known rocks. The remarkable "trap-granulites " appear related to the granular diorites and gabbros (p. 230), and their constituents are probably recrystallised products, being clear and beautifully fresh under the microscope. At the opposite extreme are the common quartzose granulites, which are granular quartzites, often with accessory minerals. Some of the Swedish "halleflintas are metamorphic products; others may be eurites or old lavas.

are

Many of Hauy's "Leptynites" (a name given because the felspar particles "thinned down" in their dimensions) are felspathic granulites; but a more precise name can often be assigned to them in the field, when they prove to be granular aplites, or even felspathic sandstones altered by contact-metamorphism.

The common microscopic character of the granulites is the occurrence, as above stated, of a fine-grained granular structure. Foliation may be also visible, but is not necessarily distinct so far as sections are concerned.

287

PART IV.

THE EXAMINATION OF FOSSILS.

"The search for a fossil may be considered as least as rational as the pursuit of a hare."-WILLIAM SMITH, Stratigraphical System of Organized Fossils, 1817.

"Neque mirandum in mediterraneis, et montibus altissimis reperiri animalia maritima in lapides conversa, non enim absurdum est, ubique mare extitisse."-CAESALPINUS, De Metallicis, 1596.

CHAPTER XXI.

INTRODUCTORY.

THE exact determination of fossil species is a matter rather for the specialist than for the student or the geologist in the field. The proximity of good libraries and continually revised museumcollections is essential for the comparison of the specimens collected with accurately defined types of species. But every

geologist should be acquainted with the principles that guide the paleontologist, and with the points on which he relies for the discrimination of the more important genera of fossil forms.

There are certain names that are household words among geologists, although possibly of little interest in pure zoology. Every reader of text-books encounters Phacops and Productus, Trigonia and Limnæa. He collects, moreover, with enthusiasm in the field, where he views, among limestone scarps or delicately bedded shales, a fauna almost in its habit as it lived. In leisure hours he endeavours to connect what he brings home with the types selected by stratigraphers. In the following pages, therefore, we propose to give an outline of the characters of the most typical and abundant fossil genera, confining ourselves to inver

tebrates and mainly to forms well known in the British Isles. Prefixed to the account of the members of each class is a kind of glossary of the terms ordinarily used in the description of such of their parts as are found fossil. It is hoped that by this means the reality of the distinctions made between the remains of certain genera, and the reality of the relationship between others, may be adequately grasped, and that attention may be called to the features which should obtain prominence in the description of a fossil form.

Such features may or may not possess importance to the zoologist. It must be remembered that we are here examining fossils from a rather limited and geological standpoint, our first consideration being, what is the age of any series of deposits, and secondly, what were the conditions prevailing in the area under examination at the time that any particular bed was being laid down? We have examined the mineral features and have classified the rocks. The age of the deposits is a matter of paramount interest and importance, and the species of fossils are our surest guide. The assemblage of genera will, however, be of very considerable service in the absence of means of accurately defining species; and this assemblage, moreover, will generally answer the question as to the prevalence of fresh-water or marine, shallow-water or deep-water conditions.

Hence, while the philosophic zoologist may be inclined to think lightly of elaborate specific distinctions, the geologist has to consider animals and their remains from a position peculiarly his own. While a geologist looks vertically down each long chain of life-forms that has yielded us an existing species, the geologist endeavours to look horizontally across all the lines at once, cutting, indeed, the complex structure of chains, continuous or bifurcating, with a plane that comes in contact merely with the contemporaneous links.

Moreover, he has to deal almost entirely with the hard parts of his animals, or with the mere leaves of plants dissociated from reproductive structures. His fossils must be classified largely by conjecture, and often by means of characters, as we have pointed out, of doubtful zoological value.

In this little book we deal purely with the relics known as fossils, not by any means ignoring that most fascinating science, palæontology, but looking merely at one branch of it, which we might term "sclerography," the description of hard parts only. Questions of life-history, animal structure, or relationship with modern forms, we must leave to zoological and palæontological writers; and we need scarcely add that an acquaintance with

such matters is pre-supposed in every serious student of fossil remains. The worker who would, for the purpose of an examination or the labelling of a collection, go through the following pages without looking beyond them, without endeavouring to picture a fossil as part of a living moving fauna, may acquire a number of disconnected facts, but will scarcely be in a position to apply any one of them to the explanation of stratified deposits in the field.

The order in which the classes of organisms are here arranged is purely utilitarian. The Hydrozoa, Actinozoa, and Polyzoa, have thus been treated in succession, owing to the difficulty sometimes experienced in correctly assigning a fossil colonial organism to any one of these divisions. The Lamellibranchiata follow on the Brachiopoda, so that the contrasts between the two groups of bivalve shells may be emphasised. The Echinodermata and the Annelida thus obtain a somewhat late position; but even zoologists will hardly object to the splitting-up of the heterogeneous group of "Vermes."

The genera selected are arranged simply under their respective classes, and occasionally orders, without division into families. The alliance between any two or more forms that may be discussed is, however, pointed out, and a black line between two descriptions marks the passage to a fresh group of types. Such descriptions as are here given cover only a small portion of the ground, and, when a specimen under examination fails to agree in essential features with any of those quoted, its characters should be written down, and comparison made at the first opportunity with examples in museums and with the details in special works, such as the volumes below mentioned or the publications of the Palæontographical Society. Practice in describing fossils will naturally develop greatly the observation of their essential structures.

The division of fossil forms into marine, brackish-water, or fresh-water, is naturally difficult in some cases; and to reason from analogy with modern forms is likely to be misleading. But, from the association of one fossil genus with another, and from the physical characters of the strata in which they lie, we can add information on this important point with confidence in the case of most of the common forms about to be discussed.

WORKS ON ANIMAL PALEONTOLOGY.

F. BERNARD.-Éléments de Paléontologie. Baillière, Paris, 1895. Very well illustrated. See note to ZITTEL below.

On the

P. FISCHER.-Manuel de Conchyliologie. Savy, Paris, 1887. lines of Woodward's work, extended and brought admirably up to date. HOERNES.-Elemente der Palaeontologie (Palaeozoologie). Veit. Leipzig, 1884. (Also a French edition by Dollo; pub. by Savy, Paris, 1886). A handy one-volume work.

NICHOLSON AND LYDEKKER.-Manual of Palæontology. Blackwood, 1889. 2 vols.

PHILLIPS.-Manual of Geology. (Physical Geology, Stratigraphy, and Palæontology.) Edited by R. ETHERIDGE and H. G. SEELEY. C. Griffin & Co., London. 2 vols.

QUENSTEDT.-Handbuch der Petrefaktenkunde. Laupp, Tübingen, 1885.

2 vols.

STEINMANN.-Elemente der Paläontologie. Engelmann, Leipzig, 18881890. (Especially clear figures.)

H. WOODS.-Elementary Paleontology; invertebrate. Cambridge Univ. Press. 2nd ed., 1896.

S. P. WOODWARD.-Manual of the Mollusca. 1st edit. pub. by Weale, 1851-6. Now pub. by Crosby, Lockwood & Co. Includes modern forms, and is in many respects a classic.

A. SMITH WOODWARD.-Outlines of Vertebrate Paleontology. bridge Univ. Press, 1898.

Cam

ZITTEL. (i.) Handbuch der Palaeontologie. Oldenbourg, Leipzig, 1876 and onwards. (French edition by Barrois; pub. by Doin, Paris.) 4 vols. (ii.) Grundzüge der Palaeontologie (Palaeozoologie). Ibid., 1895. This involves many revisions of points in the previous "Handbuch," and like the work of Bernard, forms one of the most comprehensive works in one volume extant. A considerably modified English version by C. R. Eastman is now appearing (Macmillan, vol. i., 1900).

MODE OF OCCURRENCE AND PRESERVATION OF FOSSILS.

Some rocks, from their mineral and physical constitution, are admirable preservers of fossils, while others contain few or, perhaps, none. In this latter case diligent search must be made for casts and impressions, and concretions of ironstone, silica, &c., must be examined for their included and protected fossils. Even if there are no fossils, it may still be possible to correlate the strata with others in which fossils are abundant.

Thus sandstones are often devoid of fossils for various reasons. Apart from their permeability, the coarser grits and the conglomerates would be likely only to contain fragments, since the materials would grind delicate shells to pieces during the actual deposition of the rock.

Clays preserve shells excellently, but extraction when the rock is moist is almost hopeless. The dried talus at the foot of a clay-exposure, or the small rubbly lumps thrown aside in a brickpit and broken up by sun-cracks, may be turned over with great