More than thirty pounds of the hair was collected. The brain and the capsule of the eye were in a good state of preservation. The skeleton, Fig. 68, together with a large quantity of the hair, is in the Museum of Natural History at St. Petersburgh. These animals are assigned to the pleistocene period, when they appear to have been numerous in that locality. Dr. Mantell states that thousands of fossil ivory tusks are annually collected there, forming a lucrative artiele of commerce, and that the remains of a greater number of elephants have been discovered in Siberia, than are supposed to exist at the present time all over the world.* Insects occur perfectly preserved, sealed up in amber, a fossil resin. Parts of the stomach and skin of large reptiles have in a few instances been found in older rocks, preserved by the antiseptic property of certain salts in the rocks. The "eatable earths" which the inhabitants of some countries have eaten mixed with saw-dust, consist of fossil infusoria. Usually, the harder parts of animals, the bones, shells, and crustaceous shields only, have been preserved.

125. In some cases no part of the animal or plant is preserved, but the space which the body occupied having been emptied by its decay is filled with mineral matter infiltered, and thus presents a perfect cast; or if mineral matter has not been infiltered we have only the mould. In a few instances impressions of only a part of the body, as foot-prints, are found. The tracks of birds occur in the New Red Sandstone above the coal, Fig. 69, though their skeletons have not been found below the chalk. Similar traces of other animals are met with in the same sandstone. 126. Petrifaction consists in the substitution of mineral

* Wonders of Geology, sec. II. & 17.

for organic matter. In some instances the animal or vegetable substance is almost entirely removed, while the organic structure is retained, so that thin sections examined with the microscope, show the forms of all the fibres and vessels Fig. 69.

Bird-tracks in New Red Sandstone.

in their proper places. Limestone fossils placed in acids, have had all the petrifying material dissolved, and yet exhibited the animal tissues in a perfect form. The process of petrifaction has been imitated artificially; bones, leaves, &c.

Fig. 70.

Tracks of the Cheirotherium.

have been buried in mud and sand, and after the lapse of a few years have been found petrified. The process is influenced by the presence of salts, as the sulphate of iron, in the mud, and is accelerated by heat and pressure. All fossils, however, are not petrified; nor does the age of the rock containing the fossil necessarily determine the amount of petrifaction: bones have been obtained from the Wealden, that were light and porous, while some from the most recent tertiary rocks were completely petrified.

127. Plants are sometimes petrified, but have more frequently sustained chemical changes, by which their own elements have been transposed and their vegetable structure destroyed; subjected to moisture and pressure, secluded from the air they ferment, evolve heat, and are converted into bitumen. This change is partially illustrated by a mass of half-dried hay, which ferments, becomes of a black color, and sometimes generates sufficient heat to take fire. Bitumen is a black combustible substance, and liquid as petroleum, naptha; viscid as asphaltum, mineral pitch; or solid as jet, cannel and bituminous coals. In like manner animal muscle, buried in wet earth from which the air is excluded, is converted into a fatty wax called adipocere, (adeps, fat, and cera, wax,) retaining no trace of the original muscular fibre. In some of the fishes of the Old Red Sandstone, their muscles, blood, &c., have been converted into a dark-colored bitumen, which in some places pervades the rock to such a degree as to cause it to be mistaken for coal.

It resembles black wax, or when fluid, the coal tar of the gas works. This animal bitumen is eminently antiseptic, preserving in all their elasticity the bones, fins, and scales enveloped in it, better than the oils and gums applied to the old Egyptian mummies.*

128. The petrifaction of animal and vegetable bodies is frequently accomplished by means of the metals. The metallic salts, the sulphate of iron for example, dissolved in the waters of the earth, are decomposed; their oxygen uniting with some of the elements of the organic bodies, the metals are precipitated as sulphurets. Hence fishes are frequently found incrusted with iron-pyrites (sulphuret of iron) while their internal parts are converted into stone or bitumen; not unfrequently, however, their whole substance is changed into metal, all the traces of organic structure obliterated, and their form only preserved..

129. The most common petrifiers, are carbonate of lime, oxide carbonate or sulphate of iron, and silica. A fossil petrified in limestone, however, will not necessarily be calMany of the fossils of the chalk are flints, and The cavities, as the interior

careous.

often filled with crystals of

those of clay-slates, calcareous.
of shells and hollow-bones, are
limestone, or of silica, which, dissolved in water, was infil-
tered into these closed cavities through the pores of the
shell or bone.

130. The means requisite for determining the characters of fossils, are furnished by such a degree of knowledge of botany, zoology, and comparative anatomy, as is adequate to the determination of living species. The same modes of investigation apply to both fossil and living species; they

are the natural complements of each other. Systems of botany and zoology are not complete without these extinct forms. In consequence of the relations which exist between the parts of animal frames, the observer is enabled to determine from a single bone the form and position of the other bones, and the entire condition of the animal. Thus the sharp, retractile, curved claws of the lion require that the bones immediately above them on the foot, and with which they articulate, be of such a shape as will allow free motion; the bones preceding these require also to be adapted to the design intended to be subserved by the claws. With these claws are associated the pointed sharp teeth adapted to tearing and cutting flesh. In animals feeding exclusively on flesh we always find the intestines about one fifth as long as in herbivorous animals. Hence a single tooth, or claw, suggests to the comparative anatomist the general form and habits of the animal. A single tooth of the Iguanodon enabled Cuvier to decide that the animal to which it belonged was an herbivorous reptile. The scales of fishes are so highly characteristic that Professor Agassiz has made their peculiarities the basis of his classification of these animals. A single scale found in the intestines of an Ichthyosaurus enabled him to identify the extinct species to which it belonged. In this way, peculiarities of structure reveal to the paleontologist the characters and modes of existence of creatures that ceased to exist ages before the creation of man, and to the geologist the condition of the world at that period.

131. The relics of animal and vegetable organization occur in almost every stratified rock, but in much greater numbers, and in a better state of preservation in some strata than in others. Nor are they equally abundant throughout