carried on in the most exquisite detail.  When wood, for example, is undergoing petrifaction, the woody tissue may be replaced, particle by particle, by silica in solution through the action of underground waters, even the microscopic structures of the wood being perfectly reproduced.  In shells originally made of Aragonite, a crystalline form of carbonate of lime, that mineral is usually replaced by calcite, a more stable form of the same substance.  The most common petrifying materials are calcite, silica, and pyrite.

Often the organic substance has neither been preserved nor replaced, but the form has been retained by means of molds and casts.  Permanent impressions, or molds, may be made in sediments not only by the hard parts of organisms, but also by such soft and perishable parts as the leaves of plants, and, in the rarest instances, by the skin of animals and the feathers of birds.  In fine-grained limestones even the imprints of jellyfish have been retained.

The different kinds of molds and casts may be illustrated by means of a clam shell and some moist clay, the latter representing the sediments in which the remains of animals and plants are entombed.  Imbedding the shell in the clay and allowing the clay to harden, we have a mold of the exterior of the shell, as is seen on cutting the clay matrix in two and removing the shell from it.  Filling this mold with clay of different color, we obtain a cast of the exterior, which represents accurately the original form and surface markings of the shell.  In nature, shells and other relics of animals or plants are often removed by being dissolved by percolating waters, and the molds are either filled with sediments or with minerals deposited from solution.

Where the fossil is hollow, a cast of the interior is made in the same way.  Interior casts of shells reproduce any markings on the inside of the valves, and casts of the interior of the skulls of ancient vertebrates show the form and size of their brains.

Imperfection of the life record.  At the present time only the smallest fraction of the life on earth ever gets entombed in rocks now forming.  In the forest great fallen tree trunks, as well as dead leaves, decay, and only add a little to the layer of dark vegetable mold from which they grew.  The bones of land animals are, for the most part, left unburied on the surface and are soon destroyed by chemical agencies.  Even where, as in the swamps of river, flood plains and in other bogs, there are preserved the remains of plants, and sometimes insects, together with the bones of some animal drowned or mired, in most cases these swamp and bog deposits are sooner or later destroyed by the shifting channels of the stream or by the general erosion of the land.

In the sea the conditions for preservation are more favorable than on land; yet even here the proportion of animals and plants whose hard parts are fossilized is very small compared with those which either totally decay before they are buried in slowly accumulating sediments or are ground to powder by waves and currents.