variable temperature, we should expect it to be much of the same kind, the supply being derived laterally from the same reservoir which supplied the springs between c and d, and the impervous bed d (impervious so far as regards the ready passage of water through it) preventing appreciable communication with, and circulation of, waters of a higher temperature beneath. Should waters find their way, as springs, by means of joints or fissures, from the reservoirs in both porous beds, a and c c, beneath the line of variable temperature, more rapidly in some places than in others or the beds themselves differ materially in the facility with which water can pass through-variations may be expected, important or not, according to circumstances, in the temperature of springs issuing from them. All other things being equal, the lower reservoir assuming that the temperature increases from the surface downwards—would be expected to supply the water with the more elevated temperature. It becomes needful, therefore, that after other conditions have been ascertained, the quantity of water delivered by a spring in a given time, and the rapidity with which it flows, should be duly regarded.

With respect to the temperatures of those waters which, in limestone districts especially, rush out, often in considerable volume and with much force, from subterranean channels, and which result from the loss of many minor streams and of rain-water amid fissures and cavernous rocks, they may be often very deceptive. Should the waters have been absorbed partly as streams, previously exposed to the temperature of the climate of the region, and partly derived from slow percolation through chinks, joints, and the minor cavernous structure of the rock, a mixed heat would follow, affording no correct data as to the temperature of the subterranean channels through which the waters have passed. When, also, the whole is derived from the absorption of atmospheric waters by channels of various kinds, the rapidity of passage of the waters downwards to the great drainage stream, and the differences in this respect have to be considered, as also the chances, not uncommon in some districts, that great fissure waters, derived from considerable depths, may not be mingled with the general volume of those discharged. Hence much care is required in investigating the temperatures of waters thus discharged, however desirable it may be that they should be properly ascertained.

While on the one hand the observer has to regard the adjustment of water, permeating amid the fissures and joints, or the mass of rocks, to its greatest density, and the variable mechanical manner

in which this may be effected, he has also to consider the depths at which water itself may cease to exist; assuming the increase of temperature from the surface downwards, whatever its rate, locally or generally, to be certain, as the general evidence would lead us to believe. Should it be inferred that the rate of increase of heat usually supposed probable, namely 1° Fahrenheit for each 50 to 60 feet, is too great, and that sufficient information as to this rate has not yet been obtained, if we take only 1° for every 100 feet, we still seem to obtain a comparatively minor depth, allowing for increase of pressure from the superincumbent water, with the friction on the sides of any fissures, for that portion of the earth's crust in which water may be considered to circulate under the most favourable conditions. Taking the ordinary mode of calculation, allowing for pressure at increased depths, and assuming every facility of movement of the waters in a fissure, it may be estimated that at a comparatively moderate depth steam would be found instead of water.

Waters in fissures, rushing upwards with a rapid rate of outflow and in considerable volume may (as noticed p. 19) bring with them a greater temperature than those finding their way upwards with less velocity and in smaller quantity, the one heating the waters communicating with them laterally in their course upwards, beyond the temperature due to the containing rocks themselves, and the ordinary percolation of water through them; the others being cooled by these lateral waters. In certain districts, such as those where volcanic fires have once found vent, and which may be now concealed by various overspreading aqueous accumulations, there may be influences of this kind much modifying the exact depths at which certain temperatures would otherwise be found. No doubt, supposing a general source of heat to exist in the earth governing the outer temperature of its crust on the great scale, these would be merely local variations, yet, when endeavouring to ascertain the distribution of heat in the globe, all such variations require attention, so that the disturbing circumstances may be duly separated from the essential causes of the increase of heat downwards from its surface.

CHAPTER XXVI.

SEA

MODE OF ACCUMULATION OF DETRITAL AND FOSSILIFEROUS ROCKS.-DETRITAL ROCKS CHIEFLY OLD SEA-BOTTOMS.-MIXTURE OF BEDS WITH AND WITHOUT FOSSILS. VARIABLE MODE OF OCCURRENCE OF ORGANIC REMAINS.BEACHES OF THE SILURIAN AND DEVONIAN PERIODS OF THE NEW RED SANDSTONE TIME, MENDIP HILLS, ETC.—OF THE LIAS PERIOD.- VARIED MODES OF OCCURRENCE OF THE LIAS. -BORING MOLLUSCS OF THE INFERIOR OOLITE TIME.-OVERLAP OF INFERIOR OOLITE, MENDIP HILLS.LIAS CONGLOMERATE PIERCED BY BORING MOLLUSCS.- LAND OF THE TIME OF THE LIAS.-EVIDENCE OF LAND FROM FRESH-WATER DEPOSITS.-EFFECTS PRODUCED ON COASTS, RIVERS, AND LAKES, BY CONTINUED ELEVATION OF LAND ABOVE SEA.-ELEVATION OF LAND OVER A WIDE AREA.-EFFECTS OF CLOSING THE STRAITS OF GIBRALTAR.-UNEQUAL ELEVATION OF LAND.-LAKES ON THE OUTSKIRTS OF MOUNTAINS.-MIXTURE OF ORGANIC REMAINS OF DIFFERENT PERIODS FROM SUBMERGENCE OF LAND.-VARIABLE EFFECTS OF SURMERGENCE OF PRESENT DRY LAND.

THE observer, having well considered the manner in which the accumulations of mineral matter are at present effected, chemically and mechanically, through the agency of water, as also the mode in which the remains of animal and vegetable life may be entombed amid such accumulations, has to study the various layers, beds, or other forms of mineral substances formed by aqueous means, and in which organic remains are more or less distributed in various parts of the world. In one respect he has an advantage over his previous investigations, inasmuch as while he could then often only infer that which takes place beneath seas and lakes, he has in these rocks frequent opportunities of obtaining direct evidence of that which actually occurred beneath them, the large proportion of these beds being the bottoms of various seas or bodies of fresh water, deposited over each other, and subjected to variation from local causes.

Inasmuch as the dry land of the world is thus little else than the bottoms of seas and lakes, intermixed with igneous matter vomited upwards at different times from beneath the surface of the earth, some of the latter spread at once on this surface, at other times only laid bare by the removal of superincumbent deposits, the

observer will have to dismiss from his mind the existing dry lands and waters of the world and substitute such other distributions of them as may best accord with the evidence which, from time to time, he will obtain. No matter how highly raised into mountains, or slightly elevated in plains, these ancient bottoms of oceans, seas, and bodies of fresh water may now be, they did not constitute dry land when formed, and consequently waters once occupied the areas where they now occur. We have seen that, to produce detrital accumulations, certain conditions of dry land are needed, whence their component parts have to be derived; and, therefore, to form the ancient sea-bottoms of any given time, dry land appears required out of an area so circumstanced, and yet so near to it as to afford the materials found. Considerations of this kind demand an enlarged view of the physical geography of different geological times, and such a disregard of the existing distribution of land and water that while all due weight is allowed for the employment of a given amount of mineral matter, over certain large areas, in the production of detrital accumulations of different dates-the wearing away of one portion raised above the ocean presenting materials for an equal and subsequent deposit beneath it in an adjacent situation; and consequently, that oscillations in the relative levels of the existing areas of our present continents may keep such matter much in one large area, the mind of the observer must not be too much occupied by the present arrangements of land and water on the surface of the earth.

While evidence is sought amid detrital or fossiliferous accumulations, of the mode in which the mineral matter of rocks has been chemically or mechanically gathered together, and the observer endeavours to trace among them former beaches, estuaries, bays, promontories, shallow and deep seas, fresh-water lakes, and the other modifications of water around and amid dry land, he has at the same time most carefully to study the mode of occurrence of any organic remains found in these accumulations. He will have to see if there be evidence that the animals or plants lived and died in or upon the beds where their remains are now found; or whether, after death, such remains were drifted into these situations. He will also have most carefully to refer to the distribution of the animals and plants existing at any given geological time, according to conditions, regarding that distribution as well on the large scale as with respect to any minor area.

With respect to the class of rocks usually named fossiliferous, this term has to be regarded in an extended sense. It is by no

means required that the various beds composing any given series of sea-bottoms, should all contain organic remains in certain localities. Frequently, as in the subjoined sketch (fig. 163), representing a series of beds of rock, a b c d and e, exposed on a cliff, one of them only, such as d, may contain them, the others Fig. 163.

not affording any animal or vegetable exuvia. These beds are not, however, the less interesting on that account, inasmuch as some cause for this difference may present itself by diligent investigation, of importance as bearing upon the conditions, or their modifications, under which the whole series may have been formed. Should the beds be of different substances-as, for example, should a b and e be formed of sands of different kinds consolidated, as hereafter to be noticed, into sandstones; e, of gravels now hardened into a conglomerate; and d be composed of mud, now constituting a shale; the mode of accumulation of the non-fossiliferous beds have to be studied, as well on the small as large scale; and this study may tend to show how it probably occurred that the mud contained the remains of life, which has existed on or in this seabottom of the time, while no such remains are found in the sands and gravel.

Some rocks are only seen to be fossiliferous at rare intervals, a depth of perhaps only two or three inches affording organic remains, these occurring amid a great mass of mud, silt, and sand, as, for example, among the lower of the oldest fossiliferous deposits, the Silurian, a class of rocks for the due appreciation and knowledge of which geologists stand so much indebted to Sir Roderick Murchison. In certain parts of this series, as developed in the British Islands, there are hundreds of feet in depth, in some localities, where no trace of an organic remain is found, and then a thin seam, replete with the remains of animal life, may be seen, showing that the portion of the sea-bottom which it represents was