basis of their succession in chronological sequence. Although the relative position of the strata is the only infallible mark of time-sequence, it is the fossils in the strata that are the only infallible marks of time-periods.

Nomenclature of the International Congress of Geologists.-In general usage the time-designations have been applied directly to the formations, as in the nomenclature proposed by the International Geological Congress, where the formation-names stage, series, system, group, have their corresponding timenames age, epoch, period, era. In a similar way various other terms, which apply to the strata of formations, have their corresponding terms for the fossils of such formations.

Fauna and Flora. A particular bed, stratum, or formation is said to have its fauna or flora, in the same way as a particular geographical region or province has its fauna or flora. A particular rock stratum marks a particular faunal horizon, as the Tully limestone may be called the horizon of the Cuboides fauna. We find an admirable definition of fauna in the Century Dictionary: "Fauna, the total of the animal life of a given region or period; the sum of the animals living in a given area or time." Flora is used similarly for the plants of a region or period.

Horizon. We find under the word horizon an equally apt definition of that term. A geological horizon is defined as "A stratum, or group of strata, characterized by the presence of a particular fossil, or a peculiar assemblage of fossils, not found in the underlying or overlying beds."

Zone and Stratum.-The term zone is applied in geology to the stratum or the strata in which a particular fauna or flora is distributed. In some cases authors speak of the zone of a particular species; but whether a single species, or that one and other associated species, be taken as the distinguishing marks of a geological zone, the difference between a zone and a stratum is found in the distinction that the zone is characterized by continuity of the same life and the stratum by continuity of the kind of stratified deposit.

Facies. The term facies is used in a particular sense in geology to apply to the particular composition or condition of a formation in a given region; for instance, the Hamilton

formation in western New York is calcareous and finely argillaceous; in eastern New York the same formation is arenaceous and flaggy; although representing the same formation, one may be called the argillaceous or calcareous facies, and the other the arenaceous facies, of the Hamilton formation.

Area, Province, Region.-Again, the terms area, province, region, when applied geologically, refer to the geographical districts in which there was greater or less uniformity in the kind and succession of sedimentation for a given geological period. Thus, the Appalachian province or the Mississippian province may be spoken of. These same terms when used in zoology or botany refer to the districts which, separated by more or less sharp physical boundaries, are characterized by distinct faunas or floras.

Geological Range and Geographical Distribution.-A convenient distinction may be drawn in the usage of the terms range and distribution, which are now almost synonymous. In speaking of the separation of species, or genera, or faunas, or floras, when separated in space, distribution will be used; when separated in time, range. Thus, according to Ulrich, the Vitulina fauna of the Middle Devonian may be said to have a distribution limited to South and North America and Africa; its range is Lower and Middle Devonian.

Variations and Mutations.-Waagen has proposed to distinguish the changes of form observed on comparing the same species from different places. When the specimens compared belong to the same geological horizon, but come from the same or different geographical areas, the differences of form are called variations; when the specimens come from different geological horizons, thus representing time-range, the differences of form are called mutations.

Development and Evolution.-Another analogous distinction, which is explained more fully elsewhere, is observed in the restriction of the term development to the processes of expansion of characters of the individual in ontogenetic growth, and the term evolution to the changes expressed in the individuals succeeding each other in phylogenetic succession.

Initiation and Origin.-Another distinction, in the way of greater precision, is in the use of the term initiation in place of

origin, when speaking of the first appearance of a new type of structure in the geological formations. It is difficult not to associate some idea of causation with the terms origin and originate, but the term initiation refers simply to an incoming or a beginning to appear, leaving other questions open for discussion.

System. This is the name for one of the larger geological divisions, but there is no uniform rule for its application. Originally, as proposed by Murchison, system was applied to a series of rocks continuously exposed in some geographical region. Thus, Silurian system was the series of rocks exposed in Wales and western England at one time inhabited by the Silures. The Devonian system was the series of rocks exposed in south and north Devonshire; Permian system, certain fossiliferous rocks first thoroughly studied in Perm, Russia; etc. The term system was afterwards adopted as a name for a large and prominent series of stratified rocks, as Carboniferous system, Tertiary system, etc.

Systems have been arbitrarily determined, and the list as given. including those in which fossils have heretofore been found, is as follows: Cambrian, Ordovician, Silurian, Devonian, Carboniferous, Triassic, Jurassic, Cretaceous, Tertiary, and Quaternary or Recent, or including Recent. These, as has been said, are arbitrarily fixed, and there is in each case a typical system expressed in the rocks of a particular region.

These systems are applied with an approximate degree of uniformity in all countries, although arbitrarily; and era is the time-designation which is applied to indicate the lapse of time during the formation of the rocks of a single system.

Geographical Conditions Determining the Local Characters of Stratified Rocks.-There are a few particulars, regarding the way in which these rocks were formed and their present condition and order, which help to explain the conditions under which the organisms lived in the past, and may explain why we have full records in some cases, very little record in others, and in many cases very sparse and greatly broken records of the life-histories we are seeking to read.

The stratified rocks, as already stated, are the result of water-action: First, erosion from already formed rocks; sec

ond, transportation of the fragments by water; and, in the transportation, third, separation of fine from coarse and further rounding of the individual grains; fourth, sedimentation under water in layers or strata. The materials for each stratum have gone through these various processes of wateraction. The result is that the present characters of the strata have been determined by (a) the nature of the source of materials, (b) the rate, direction, and force of the activity of the water, and (c) the relations of the bottom of the ocean to the surface, or the depth of the water. Each of these three conditions is variable and generally is the same for only a limited area. To illustrate: We know from observing the phenomena of an ocean beach that the beach material where the shores are low and composed of soil is made up of the wash of the shore. If a large river empties in the vicinity, the shore is made up of fine silt and mud; if, on the other hand, the shores are hard rocks, the beach is composed of coarse pebbles and gritty sand, the result of the disintegration of the rocks themselves. If we examine the shore material of Florida, where calcareous rocks alone are exhibited, we find the sand composed of broken shells and corals. This, when filled by deposited calcite carried into the interstices in solution and hardened, becomes a calcareous rock, called coquina, and finally a compact limestone.

Again, if we examine the materials lying on the beach at high tide and those on the bottom out to a depth of a hundred fathoms, we find that the coarse pebbles and boulders. are distributed along the line of most violent wave-action near shore, then gravel, and further out only fine sand, and finally only the finest silt appears. This sorting is entirely co-ordinate with the change in violence and rapidity of normal motion of the water in waves and currents. The more rapid and violent the motion of the water, the larger the particles moved and transported by it, and, hence, the farther out from its source the material is borne, the finer and less in amount will be the resulting deposit.

For all fragmental material the land surface, where it comes in contact with water in motion, may be regarded, in a general sense, as the source, and, in a general way, distance

from such source determines the relative size of the particles making up the sediment. The source may be far up in the interior of the continent where river erosion or lake erosion is eating away the land, or it may be on the ocean-shore, but in general it is true that local geographical conditions are fundamental in determining the lithological character of geological formations.

Varying Conditions of Environment in Relation to Timeestimates. The conclusion from these observations is that all sedimentary rocks may be supposed to have been formed within about a hundred miles of the shore from which the sediments were derived. This theory is supported by the deep-sea soundings, which show very small amount of material accumulated on the bottom of the present ocean at great distances from land. From these considerations we turn to our classification of formations, and see why it is that we cannot expect to find uniformity of details in either the structural or stratigraphical order, or in the lithological composition of the formations. (1) At the same time there may be in process of formation a limestone, a sandstone, a conglomerate, and a mud-shale, and all may be forming within a relatively short extent of coast. (2) In the same period of time the thickness of material accumulated may greatly vary; while an inch of limestone may be deposited in one place, a hundred feet of sandstone may be formed in another. Thus the limestone of one locality may be represented by a sandstone in another, and a thousand feet of strata in one place may be represented by a hundred or less in another not. far distant.

Relative Order of Deposits in Relation to Depression and Elevation Another series of facts may be considered in this place. The shore-lines do not remain constantly the same for the accumulation of sediments. The simple fact that there are marine fossils in rocks above the level of the ocean is evidence that there has been a change in the relative level of land and ocean surfaces; there has been an elevation of the land surface. Since the conditions of sedimentation vary. with the distance from shore-line, a particular series of these conditions extending from shore-line out into deep water will