TEMPERATURE. 16. THE TEMPERATURE OF THE EARTH is a subject the consideration of which exercises a most important influence on the reasonings of geologists. There is, first, the surface temperature, which affects the growth and distribution of plants and animals; second, the temperature of the crust, which may give rise to gaseous exhalations, thermal springs, mineral and metallic transformations; and, third, there appears to be an internal or central temperature, having its seat beneath the solid crust, and which seems to be the cause of volcanoes, earthquakes, and other similar phenomena. 17. Of the surface temperature, we know that it is influenced, first, from day to day, and from season to season, by the heat of the sun; second, by the degree of latitude, being warmest at the equator, and gradually diminishing towards either pole; third, by the distribution of land and water, the sea being less liable to sudden fluctuations of temperature than the land; fourth, by the nature of the surface, the kind and colour of matter variously absorbing and retaining the heat derived from the sun; and lastly, by the elevation of the land above the mean level of the sea, the more elevated being the colder regions. All these influences are at present in active force, affecting more or less the growth and distribution of animal and vegetable life; and in like manner must they have exerted themselves at former periods, though perhaps increased or counteracted by certain conditions not now existing. 18. The temperature of the crust may be affected either by heat from the sun, by heat created by chemical action among its materials, or by heat from the interior. During summer, the surface is heated by the sun, and this heat is communicated to a certain depth; during winter, it is again given off to the surrounding atmosphere more or less, according to the severity of the winter. This alternate receiving and parting with heat may differ considerably in any particular summer or winter, but over a number of years it is found to be nearly stationary; that is, the amount of heat received and given off may be said to balance each other. According to this doctrine, the earth in summer will be warmer near the surface than it is at small depths; and in winter will be colder at the surface than at depths beyond the influence of the passing cold. 19. The heat of the sun can only affect the earth to a limited depth; for, as the heat of summer proceeds downwards, it is arrested by the cold of winter, and thus conti nually kept within a given limit. By actual experiment, it has been ascertained that, at a given depth beneath the surface of the earth, there is a point at which the temperature remains constantly the same, being uninfluenced by any causes which affect the surface. This depth will vary ac cording to the kind of material of which the crust at any given place is composed, be these materials rocks, sand, or water; but in no instance has it been found to be less than 60, or more than 100 feet. A series of these depths, as represented by the white line in the subjoined diagram, is called by geologists the stratum of invariable temperature. Section, showing the stratum of invariable temperature, and relative 20. Proceeding beyond this invariable stratum, towards the centre (c), it has been found that the temperature gradually increases; a circumstance which attracted the attention of philosophers more than a century ago. In 1802 D'Aubuisson revived the investigation, and since that time, observations have been made in the principal mines of Europe and America. The greatest depths at which experiments have been conducted, are 1713 feet in Mexico, 1584 in England, and about 1300 in Germany; and in all of these the temperature has been found to increase according to the depth. In 1827 M. Cordier published a memoir on this interesting subject, in which he collected the observations of others together with his own; and having made allowance for the heat arising from the breathing of miners, for the combustion of lamps, and communication with the atmosphere, he drew the following general conclusions:-1. Below the invariable stratum, the temperature at any given depth remains perfectly constant for several years; 2. That below the invariable stratum the temperature goes on increasing with the depth; and, 3. That, taking the average of observations, this increase of temperature goes on at the rate of one degree of Fahrenheit's thermometer for every 45 feet. Others have allowed 60 feet for the rise of one degree; but even taking this lower estimate, it must follow, if the increase go on in the same ratio, that a temperature equal to 100 degrees of Wedgewood's pyrometer would be found at the depth of 160 miles. But, 100 degrees of Wedgewood is sufficient to keep in fusion any of the known rocks, so that, according to this estimate, the solid crust of the earth cannot be more than 60 or 80 miles in thickness-a mere fractional film of the distance from the surface to the centre. 21. Of the internal or central heat of the globe, we know nothing by actual experiment; but are left to infer as to its amount from the descending increase of temperature observable in the solid crust, and from the occurrence of hot-springs, vapour fissures, and volcanoes. That the heat of the crust increases as we descend, has been fully established by experiments in mines, in Artesian wells, and in the waters of other deep-seated springs; and if this temperature goes on increasing at the ratio above-mentioned, then the interior parts must be heated to an enormous degree; so much so, indeed, as fully to counteract that law of compression formerly adverted to, and which would render all known matter, if placed at the centre, so dense as to be inconsistent with the mean density of the globe. 22. Taking all circumstances into account, the following conclusions seem warrantable :-first, that the interior parts of the earth are heated to intensity; second, that this heat is the apparent cause of volcanoes, hot-springs, and other thermal phenomena; third, that the solid crust derives part of its heat from this source; fourth, that this solid crust has partly been formed by the cooling of an original igneous mass; fifth, that if volcanoes, hot-springs, &c. take place at the expense of this internal heat, the globe must be gradually cooling; but, lastly, that from the bad conducting nature of the rocky crust, this gradual refrigeration is not perceptible within any given time. EXPLANATORY NOTE. DENSITY (Lat., densus, thick)-thickness or compactness. Density is a comparative term; gold, for instance, being denser than iron, iron than granite, granite than sandstone, sandstone than water, and water than gas; that is, a cubic inch of any one of these bodies would differ in weight from a cubic inch of any of the others. To render this idea of density more definite, water at the temperature of 60 degrees has been taken as the standard or measure; hence, if water be assumed as weighing 1, the rocky materials composing the earth's crust will be 23, or two and a-half times heavier than water. ARTESIAN WELLS (Artois, a district in France)-a term applied to wells sunk by digging or boring perpendicularly through various strata, from the circumstance that this mode was first practised in the district above referred to. THERMAL (Greek, therme, heat)-warm or hot. Thermal and igneous are sometimes used indiscriminately; but it is more accurate to make a distinction. Thus, in treating of volcanoes, we speak of igneous agency; in treating of hot-springs, thermal is the more appropriate term. TEMPERATURE OF THE EARTH.-The chief of those who have conducted experiments relative to this subject are Gensanne, Saussure, D'Aubuisson, Fourier, Cordier, Quetelet, and Arago, in France, Germany, &c.; Sir John Leslie, Fox, Forbes, &c. in Britain; and Humboldt, in Mexico. Their experiments present a wonderful degree of coincidence, and are chiefly of three classes:-1. Those made in mineral veins, or in mountain masses, such as granite; 2. Those made in stratified rocks, as in coal mines; and, 3. Those made in Artesian wells and other deep-seated springs. SURFACE CONFIGURATION. 23. THE SURFACE CONFIGURATION OF THE EARTH is more the study of physical geography than of geology proper; but it is necessary to observe in what manner it influences the geological changes now in progress. The surface of the earth is extremely irregular, being diversified by hills and valleys, rivers, lakes, seas, &c. Portions of it are covered with woods and forests; other portions are elevated above the limits of vegetable or animal life, and covered with eternal snows. Some parts of the exposed surface are so hard, that no sensible decay is experienced for ages; others so soft and loose, that scarcely a shower falls without carrying away a portion to some lower level. These differences of surface material are also influenced by steepness and irregularity of position; the transporting power of streams and rivers being proportioned to the rapidity of their descent. These are familiar instances of the numerous changes effected by diversity of surface configuration; but the student has only to cast his eye over his own district, to be convinced how many geological results depend upon this cause. Those immense plains -the steppes of Northern Asia, the prairies of North and the pampas of South America-must affect and be affected in a different manner from the Himmaleh, Alps, Andes, and other mountain ranges. The showers and snows which produce torrents and avalanches on the Alps, form merely springs and harmless streams on the Apennines; and while the sluggish river is forming inland plains with its mud, the rapid torrent is carrying its burden forward to the bottom of the ocean. B 24. The surface outline of the earth is so irregular that even physical geography, with all the facilities afforded by modern intercourse, has yet presented us with a very general description; and before we can estimate the full force of this configuration as a geological agent, we must know more of the relative elevations and depressions of the land, and the nature of the rocky substances so elevated and depressed. Of the surface configuration of the world in its earlier ages, we have no positive knowledge; but we are certain that whatever it might be, it would materially influence the changes then going forward, just as the same cause is operating at the present moment. DISTRIBUTION OF LAND AND WATER. 25. THE DISTRIBUTION OF LAND AND WATER, upon which so many geological phenomena depend, is influenced by this principle alone; namely, that so long as the same quantity of water remains on the globe, a fixed amount of space will be required to contain it. If the difference between the elevations and depressions of the solid crust be small-in other words, if the hollows in which lakes and seas are spread out be shallow-their waters must extend over a greater space; and if these hollows be deep, the waters will occupy less extensive areas. The operation of this principle the student should bear in mind; for if, in the earlier ages of the world, the elevation of the land was less general, the waters would occupy larger spaces, and this more extended area of shallow water would act in various ways. It would render the climate more genial and uniform, and extending a greater surface to the evaporating power of the sun, rains and atmospheric moisture would be more prevalent. These, again, would operate on the amount and kind of animal and vegetable life on dry land; while the shallow waters themselves would be more productive of life, it being a well-known fact, that shell-fishes and aquatic plants flourish only at limited depths around the shores. Of the distribution of land and water at any former period of the world, we can only infer from the appearances which the surface and rocky strata present; but of the present distribution, we have pretty accurate information, with the exception of those regions surrounding either pole. 26. The proportion of dry land to water, as at present existing, is about one to three; that is, three-fourths of the whole surface of the globe may be assigned to water. The dry land presents itself in islands and continents variously situated; but the student has only to cast his eye over a map of the world, to observe that the greater portion is |