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FOSSIL. as may ornament the shell, and for this reason molds should always be carefully collected and treated with acid, after which the impression of the original shell surface is often shown with the utmost fidelity to detail. Another class of fossils consists of the impressions or trails made by animals crawling over the bottom of the water or over the beach, and also of burrows or casts of burrows that served as dwelling-places or passageways for worms, crustaceans, etc. The study of the footprints of reptiles and supposed birds, which are so abundant on the surfaces of the Jurassic sandstones of Massachusetts and Connecticut, was named ichnology by E. Hitchcock, who described and figured a host of such impressions. Similar footprints are found in rocks of shallow-water origin of Mesozoic and Tertiary age all over the world.

The parts of animals likely to be preserved are always those that resist longest the destructive agencies that may attack them both before and after their entombment. The soft parts are seldom preserved, and often also the hard parts are destroyed. Because of this certain groups of animals are represented by insignificant parts of their anatomy, which, though of great importance to the paleontologist, are usually laid aside by the zoölogist as of trivial interest. Thus, the presence of sponges in certain formations is demonstrated by their isolated spicules; holothurians are recognized by their minute calcareous plates and anchors; worms by their teeth and dwelling-tubes; dibranchiate cephalopods by their internal shells; and many fish by their teeth, ear-bones (otoliths), spines, and dermal scales.

The manner of entombment of fossils varies greatly. In many cases the shells of mollusks have been dead a long time, and have become incrusted with polyzoans and corals before they were entombed. In other cases they were washed along the shore and broken and worn by the waves so that now in fragmentary condition they form ‘shell limestones.’ Among the crustacean fossils we find those that were killed suddenly, perhaps by some change in the temperature of the water, in which case their remains are usually well preserved. In some rocks of fresh-water or estuarine origin certain layers are covered with the remains of fish. These evidently lived in shallow pools that were either dried up suddenly or became so heated by the sun that the fish were killed, soon to be covered by sediment. Such conditions are frequent in the Catskill, Old Red Sandstone, and Jurassic formations. Myriads of insects of Tertiary time became entangled in the soft gum of coniferous trees, and are now preserved in the amber of the Baltic and the fossil resins of Africa and New Zealand.

The old ideas regarding fossils were curious and often fantastic. A few of the Greek and Roman philosophers had well-defined ideas of their true nature as entombed animals and plants that had once lived in the sea and upon the earth; but the majority of early writers attached to them some fanciful or supernatural origin. Thus they were explained as due to the vis plastica, or creative force that formed living things out of inorganic materials; as sports of nature; as due to some peculiar fermentative process in the earth, or as originating in some unknown influence of the stars. Another hypothesis, maintained for centuries, and even now persisting in uneducated

communities, explains fossils as the remains of animals and plants washed up on the land, and there stranded by the waters of the Noachian deluge. These erroneous ideas persisted in the face of true explanations by some observers, until the beginning of 1800, when slowly the true nature of fossils and their relations to the rocks in which they are entombed began to be more universally understood, and at last during 1800 to 1840 there were laid the foundations of the science of paleontology.

For further information on the early ideas regarding fossils, consult: Lyell, Principles of Geology, vol. i. (New York, 1872); Von Zittel, History of Geology and Palæontology, translated by Ogilvie-Gordon (London and New York, 1901). For modes of fossilization and the relations between fossils and the rocks containing them, consult: Geikie, Text-Book of Geology (London and New York, 1893); White, “The Relations of Biology to Geological Investigation,” in Smithsonian Institution Report of the United States National Museum for 1892 (Washington, 1894); Marr, Principles of Stratigraphical Geology (Cambridge, 1898); Schuchert, “Directions for Collecting and Preparing Fossils,” in Smithsonian Institution United States National Museum, Bulletin No. 39 (Washington, 1895). See also  FOSSIL BOTANY. See.  FOSSIL FOOTPRINTS. See.  FOSSIL FORESTS. The popular term applied to groups of petrified tree-trunks. Such forests may be found at the locality and in the position in which they grew, or, what is more frequently the case, they may have been carried some distance from their native soil before being buried and silicified. Fossil trees are not uncommon in the coal measures of the United States; but the most celebrated examples, belonging to more recent geological periods, are those of Arizona and the Yellowstone Park. Along the Little Colorado River, in Arizona, there are great numbers of well-preserved trees, scattered over the surface, some of which attain a diameter of 5 feet, and a length of more than 50 feet. The wood-cells have been replaced by silica, which is either colorless, like quartz, or shows the beautiful tints of agate, opal, and jasper; the structure of the wood is preserved to a most remarkable degree. Heavy beds of Triassic marls cover the surface, and it is in this formation that the trees are found. The silication was probably accomplished by hot alkaline waters, carrying dissolved silica; there is evidence of volcanic activity in the region which might well give rise to thermal springs. Many of the trees have been removed for cutting and polishing into various artistic objects, rivaling onyx and the rarer marbles in delicacy of color, and this wholesale destruction has given much concern lest the forest be entirely destroyed. A similar fossil forest in the Yellowstone Valley has many erect stumps of large size. Along the shore of Chesapeake Bay, south of Baltimore, is a forest in which the giant trunks of cypress rise from a bed of peat that is covered by Pleistocene clays. The Bad Lands of the Little Missouri abound in petrified trees which have been washed out from shales and sandstones of the Laramie group. Another forest, remarkable for the great size of its trees, is found in