Geology, Vol. 1 [of 3]

by Thomas C. Chamberlin; Rollin D. Salisbury

The contribution of the Protozoa.—The Protozoa are related to the animal kingdom much as the Thallophytes are to the vegetable, and the two bear a close structural resemblance to one another. So near, indeed, do the Protozoa and the Thallophytes approach one another in their minuteness and simplicity, that the place of not a few organisms is in doubt, and the two kingdoms, in general so different, seem here to blend in the group Flagellata. The Protozoa are usually very minute one-celled organisms with very little differentiation of tissue or organs. Of the four classes of Protozoa, only one, the Rhizopoda, is found in the fossil state. The rhizopods secrete silicious skeletons, and calcareous, silicious, and chitinous tests of a great variety of forms, and this gives them geologic importance. The deep-sea oozes and the chalk deposits are their best-known contributions at present. They have probably played a more important rôle in the formation of ordinary limestones and silicious silts than can be demonstrated, because of the delicacy of their relics and the ease with which these are pulverized by wave-action in the shallow seas, or changed by recrystallization or by concretionary aggregation. The globigerina oozes are formed largely from the calcareous shells of Foraminifera ([Fig. 351]), one of the orders of rhizopods, among which the genus Globigerina is a leading form. Those forms which make the deep-sea oozes live, not on the bottom, but near the surface of the open sea, and on the death of the organisms, the shells, tests, and skeletons sink to the bottom. Chalk is formed in a similar way from calcareous Foraminifera, but not necessarily in very deep water. Foraminifera live in shallow water as well as in the open sea, and in this case they sometimes creep on the bottom or are attached to algæ, but their deposits in shallow water are usually much obscured by other kinds of deposition and by destructive action. Some of the foraminiferal shells are divided into chambers and assume various spiral forms, of which the Nummulites, named from their resemblance to coins, are notable examples. These formed an important part of the nummulitic limestone of the Eocene period.

The radiolarian ooze is characterized by the silicious tests of various members of the silica-bearing order, Radiolaria. The “Barbadoes earth” and “Tripoli” are notable deposits of fossil radiolarians.

The contribution of the Cœlenterata.—The Cœlenterata embrace the sponges, the coral polyps (Anthozoa), and the hydroids and medusæ (Hydrozoa). The contribution of coral polyps to the formation of limestone is most important, and is too familiar to require elaboration here. The corals range throughout nearly the whole fossiliferous series, and their development will be followed and illustrated in the historical chapters.

The sponges are widely represented by their spicules, and not uncommonly their aggregate form is preserved even in very ancient strata. Their contribution is largely silicious, but is partly calcareous. The hydroids and medusæ have left little trace of themselves in the rocks, although impressions supposed to represent medusæ are found in strata as early as the Cambrian. Certain coral-like forms, as the Millepores, Tubularia, and Stromatopora, are classed as Hydrozoa. The graptolites, delicate leaf-like floating forms, very serviceable in marking exact horizons on different continents because of their free distribution, are also classed here.

The contribution of the Echinodermata.—Under the echinoderms are grouped the crinoids (sea-lilies), cystoids, blastoids, ophiuroids (brittle stars), asteroids (starfishes), echinoids (sea-urchins), and holothuroids (sea-cucumbers). This is one of the marked groups of ancient as well as modern life, and its beautiful fossils grace every period in which life relics are well preserved. The cystoids and crinoids, and later the blastoids, were prominent in the Paleozoic ages, while the remaining forms were more conspicuous later, though early introduced. All divisions, except the holothuroids, whose softness prevented, have left a good record, as fossil records go. Their relics are chiefly calcareous, and they most abound in the limestones, some of which are largely made up of their remains, as the encrinital limestone ([Fig. 349]). They will be subjects of frequent comment and illustration in the historical chapters.

The contribution of the Vermes.—Most of the worms are ill adapted to fossilization and are not known in the fossil form. The segmental worms of the sea, the annelids, however, left some traces of themselves in tubes and borings and in tracks and sometimes by fossil jaws and teeth. They range from the earliest fossil-marked horizons onward, but seem to have always been an inferior group.

The contribution of the Molluscoidea.—This group includes the bryozoans, whose fossil products closely resemble the minute-celled corals, and the brachiopods, whose shells closely resemble those of the molluscs. Both are calcareous and make important contributions to the formation of limestone ([Fig. 350]). A few brachiopods secrete calcium phosphate instead of calcium carbonate. Both classes have a great geologic range and their fossils are valuable aids in identifying and correlating formations. Probably the brachiopods are more utilized for this purpose than any other single class. They are the symbol of conservatism and persistence, ranging from the Cambrian to the present time, and embracing some forms that have scarcely changed to the extent of generic difference in that time.

The contribution of the Mollusca.—The molluscs have also ranged from the earliest well-recorded times, and some divisions, as the pelecypods (lamellibranchs, embracing clams, oysters, etc.) and gastropods (snails, etc.), have undergone no very marked change beyond a rather ample and progressive development; but others, as the cephalopods (nautilus, squids, cuttlefish, etc.), mark out the progress of the ages by distinct and striking changes of form. Their shells are chiefly calcareous and they have contributed materially to the formation of limestone. Muddy and sandy bottoms are, however, more congenial to the pelecypods and gastropods than to the corals, crinoids, and many other limestone-forming types, and hence fossils of these molluscs frequently abound in shales and sandstones and give them a calcareous element. In sandstones, however, the calcareous matter is often dissolved out and only the casts of the shells remain. The molluscs will be much cited and illustrated in the historical chapters.

The contribution of the Arthropoda.—This group embraces the crustaceans, myriopods, spiders, and insects. The hard parts of their bodies are mainly horny or chitinous forms of organic matter, and hence their relics differ notably from the inorganic calcareous and silicious remains of most of the preceding forms. The Arthropoda did not at any time form a notable stratum of rock. Their geologic value lies chiefly in what they teach of the progress of life and its relations, and the aid they render in correlation and identification. In these respects the group is a notable one. It was represented in the early fossiliferous strata by the trilobites, one of the most interesting of all types of fossils. These were probably the most highly developed organisms of their times and give the clearest hints of the stage of psychological and sociological development that had been reached when first the record of life is opened to us. The record of the myriopods, spiders, and insects dates from the middle Paleozoic, and gives the first clear hints of animal life on the land.

The contribution of the Vertebrata.—In the vertebrates the dynamic or working organism may be said to reach its highest expression, unless it be in the flying insects, and their inorganic residue becomes relatively unimportant in rock formation. Although the greatest of all animal types in most respects, it has never formed more than trivial beds of rocks. There are occasional “bone beds,” but they are thin and limited in extent, and only partially formed of vertebrate matter. The geological importance of the vertebrates lies in the higher field of life evolution and in its mental accompaniment. Fishes excepted, the vertebrates are mainly land types, and have for their chief colleagues plants and insects. The other groups of animals are mainly, though not wholly, marine. The vertebrates have little place in the Paleozoic record, except near its close, but they dominate the Mesozoic and Cenozoic eras, and are conspicuously the master type to day.