The determination of the order of the strata, and the grouping of them in chrono logical order, were begun by Lehman (1756), and carried on by Fuchsel (1773), Pallas (1785), and Werner. Smith made the most important contribution to this subject when, in 1790, he published his Tabular View of the British strata. He showed their super position, and characterized the different groups by their peculiar fossils. The publica tion of his geological map of England (1815) may be said to form an epoch in the history of geology. Since then, the science has advanced by rapid strides; and it is not too much to expect that ere long all the chief geological features of the accessible parts of the world will be known and published.
Geology, in its restricted and usual sense, takes cognizance of the solid substance of of the earth, or rather of as much of it as is accessible to man's observation. He has not, by his own efforts, penetrated at any point more than a few hundred yards from the surface; but natural sections, and the peculiar arrangement of the stratified rocks (the key to which he has to some extent obtained), have given him an aquaintance with a greater thickness than could have resulted from his own labors. He has thus by actual observations, coupled with reasonings upon them, been able to construct an ideal section representing a depth of perhaps 10 m., or about a 400th part of the distance from the surface to the centre. He does not, and cannot with certainty, know anything of the structure or condition of what is deeper. This does not, however, prevent the attempt to know something of what is beyond ; and in making the attempt, there are many facts which serve as bases for inductions, or at least theorizings, as to the con dition of the interior of the globe. As the conclusions depend upon the balancing of evidence, upon the value given to one set of facts as set against another, they will differ according to the importance given by each individual to the one or other set of facts.
The long entertained opinion of the existence of a central heat seems to be on the whole fairly established, and upon such facts as these: 1. There is a regular and gradual increase in the temperature of all deep mines, equal to 1° F. for every 55 ft. of • descent after the first 100. 2 Deep wells have always a high temperature. This has been carefully determined in artesian wells, not only by applying the thermometer to the water at the surface which has risen from a known depth, but also by sinking the instrument to various depths. The .results have shown an increment similar to that exhibited in mines. Hot or boiling natural springs•rise through great and deep fissures.
3. Igneous rocks—that is to say rocks which have cooled from a state of fusion by heat. —invariably come from below, upwards, and thus testify to an amount of internal heat able either to retain these rocks in a state of fusion, or to convert them into a fluid con dition before their ejection. 4. Physics also contributes important evidence. This specific gravity of granite or basalt ip scarcely 3, while that. of the •earth, according to the recent experiments of Airy, is about 6f. If unopposed, the influence of gravitation would sq increase the density of the composing rock as to give a greater specific gravity for the earth than Of. There must, then, be some expansive force acting to reduce the gravity, and the only force with which we are acquainted that could so act, is heat. On the other hand, physics raises difficulties which militate against the fluid condition of any considerable portion of the earth's interior, and in these difficulties it is supported by astronomy. But although we may admit that the rate of increase of heat from the surface downwards goes on at the rate indicated by observation in mines and wells, we need not draw the conclusion that the interior is fluid below 25 or even 2,000m.; for we are ignorant of the effects of enormous pressure in altering the point of fusion.
The strict province of geology is the observed or observable portion of the earth's crust. The early geologists were uo more than geognosts—they observed and described the rock-mineralogy of districts, and thus laid the foundations for those generalizations which have raised geology to its present position. The materials of the earth's crust at first grouped together according to their composition,•structure, and origin; but •gradually it became evident that the rocks themselves occurred in groups, and that they had a particular order in nature; until at last,. all the sedimentary strata were arranged in a single continuous and chronological series, from characters drawn less from their lithological structure than from their organic contents. Both systems of classification are important—that of the geognost as well as that of the modern geologist. The one is the result, to a large extent, of work in the laboratory and the study, and may be accomplished by the examination of hand specimens; the other must be determined in the field, and only from the examination of rocks in the mass, and in their natural position. The term lithology has been applied to the one aspect, while stromatology (strOma, a layer) may with equal fitness be given to the other.