described by Dr. Mantell,* as occurring near Brighton, where one, elevated several feet above the sea, rests upon chalk, the rock of the coast, in the same manner as the beach at Nelly's Cove, Falmouth (fig 157), reposes on the old slates and accompanying beds. The beach near Brighton is backed by an ancient cliff of chalk, and, above the beach, chalk rubble, loam, &c., obscurely bedded, contain many teeth and bones of the fossil elephant, whence the name "Elephant Bed" has been given it. Rolled pieces of chalk and limestone are discovered among the pebbles, "full of perforations made by boring shells."† In this case the beach would appear to have been formed prior to, or during the existence of, the mammoth in Britain.

With regard to the fossil contents of these beaches, they afford much information as to the exposure of the coasts of the time to differences in the range of sea to which they may have been open; tidal streams and ocean currents being modified by alterations in the distribution of land and water. Professor E. Forbes informs me that the fossil shells of the raised beaches on the shores of the Clyde are, in many cases, those of species which, though still living in the British seas, present a more southern character than the molluscs now existing in them, and that they are confined to districts more southern and western than the Frith of Clyde. He thence infers a change in the direction of the currents from the south, (especially in that known as Rennell's current,) this change being probably due to the conformation of the coast lines of the time.

It is desirable, as has been done by Mr. R. C. Austen, to connect these raised beaches and elevated sea-bottoms of the same geological dates, and the submarine or sunk forests, with the present state of the seas adjoining or covering them. After carefully considering the subject, Mr. Austen shows that although the distribution of the detritus derived from the present coasts of France and England, in the English Channel, and from England and Ireland on the sea-bottom to the south of the latter, with the sediment brought down by the rivers to those coasts, is in accordance with the arrangement which would be expected from breaker and wind-wave action and tidal streams; there are, especially in the central parts of the English Channel and on the outer range of the 100 and 200 fathom soundings towards the

*Fossils of the South Downs, 1822.

† Mantell, "Wonders of Geology," 6th edit. (1848), p. 113, where a section and detailed description are given of the raised beach at Brighton, east of Kemp Town. "On the Valley of the English Channel;" Journal of the Geological Society of London, vol. vi., p. 69.

Atlantic, bare rocks, shingles and coarse ground, and the shells of littoral molluscs so occurring as to point to the submergence of former coasts and shallow water adjoining them. With regard to the "submarine or sunk forests," Mr. Austen calls attention to the necessity of not limiting their extension beneath the sea outwards to the shores where they are now discovered, but to take a more general view of them as parts of submerged dry land; one which would better accord with the coarse detritus at depths, or in situations, where existing wind-wave action and tidal streams would not transport it, and also with the remains of littoral molluscs, Patella vulgata, Littorina littorea, &c., found in similar situations. The evidence adduced shows very uneven ground outwards, especially towards the Atlantic; strewed over inwards by varied detrital deposits, partly the adjustment of existing circumstances, partly the mixed result of these and former conditions when the present sea-bottom was more elevated, even forming dry land connecting the British Islands with the continent. Without a proper chart * showing the condition of the sea-bottom around the British Islands, it would be difficult to convey a correct idea of the inferences to be derived from it; but, as illustrating a portion of this bottom, Mr. Austen remarks, that" within a distance from the summits of the Little Sole Bank, (parts of which rise to within 50 and 60 fathoms on the south of Ireland and off the mouth of the English Channel,) not so great as from the top of Snowdon to the sea, soundings have been obtained of 529 fathoms (3,174 feet); in other words, the Sole Bank rises from that level to nearly as high, and more rapidly, than does the mass of Snowdon from the sea level of the Caernarvon coast by the Menai Straits." †

The observer should consult the chart appended to Mr. Austen's Memoir, in which a large amount of valuable information relating to the sea-bottom of the area noticed is gathered together.

"The character of the greater part of the channel area," continues Mr. Austen "if laid bare, would be that of extensive plains of sand, surrounded by great zones of gravel and shingle, and presenting much such an admixture and arrangement of materials as we may observe at present over the Bagshot district of deposits; whilst along the opening of the channel there is an obvious configuration of hill and valley, and an amount of inequality equal to that of the most mountainous part of Wales."-Journal, &c., vol. vi., p. 85.

Referring to the examination of the range of the 200 fathom line from Cape Finisterre to the parallel of the Lizard, undertaken by Captain Vanhello in 1828 and 1829, Mr. Austen points out that the irregularity of soundings at this line, which runs at a comparatively short distance, as we have elsewhere remarked (Researches in Theoretical Geology, p. 190), outside that of 100 fathoms, (represented in figs. 65 and 99, pp. 91 and 261,) is far from being confined to one spot, but ranges not only southward, as shown by Captain Vanhello, but also to the northward. A reference to fig. 99, p. 261, will show that the Rockall Bank, westward of Ireland, much rescmbles an island under water; an uprise of only 600 feet would make it one.

Amid the complications which may arise in coasts where there has been gradual elevation of the land above the mean tidal level of the ocean, from the tidal differences above mentioned (p. 453), from the variable exposure to breaker action, as the shores become sheltered at one time and more exposed at another, from the amount of concealment of sea action on the surface of land caused by atmospheric influences, combined with running waters, and from unequal elevation of the land itself, the observer will, no doubt, require much caution while endeavouring to trace the line of coast of any one particular time. This will especially be the case when there have been oscillations, as there is frequently reason to conclude there have often been, during a time when the molluscs of adjoining seas continued to be much the same as now found in them.

In the Scandinavian region, where a slow rise of land (p. 439) is now taking place more on the north than on the south, and where surface changes, of no great geological magnitude, by which the land now separating the Baltic from the Atlantic could so easily convert a tideless sea into a branch of the ocean (a tide rushing up the Gulf of Bothnia and producing its effects in the same manner as is now found in the Bay of Fundy), traces of elevated ranges of coast are seen, which are the more interesting, as they serve to connect former movements of this kind with that now taking place. Respecting the evidence on this head, a very valuable summary and general view will be found in the observations of M. Élie de Beaumont on the researches of M. Bravais (connected with this subject) in Scandinavia. Shells of molluscs now found living, as littoral species, on the shores of Norway, are discovered raised 518 (English) feet above the sea in the province of Drontheim, 482 feet at Skiöldal and Hellesaön, 360 feet around Lake Odemark, and 206 feet at Uddevalla. Lines of erosion are also inferred to mark the former relative levels of sea and land on the Norwegian coasts. In Finmark traces of an ancient line of sea-coast were followed from Alten Bay to Hammerfest. These consisted of beaches and worn lines of rock, forming the section of a plane so inclined that while on the south of Altenfiord it rose 221 feet above the sea, it descended to 94 feet near Hammerfest. Beneath this first line was a second, 88 feet above the sea in the former locality, 46 feet at the latter, both these lines falling from south to north, the reverse of the movement now taking place in northern Scandinavia. M. Bravais

* "Comptes Rendus," vol. xv., p. 817 (1842). Report on the Memoir of M. Bravais, Voyage de la Commission Scientifique du Nord en Scandinavie, en Laponie, &c.

considers that an intermediate line of ancient coast occurs between these more marked lines, which are not exactly parallel with each other, though they may appear so for short distances, showing the observer the necessity of exact measurements in researches of this kind.

With reference to the erosion of rocks in connexion with raised beaches in an oceanic situation, and where sea levels are not likely to have been much disturbed by changes, altering tidal action during the amount of elevation of land inferred, attention may be called to one of the earliest observations of this kind by Captain Vetch, at the Island of Jura, Hebrides. He there found six or seven lines of raised beaches, the highest about 40 feet above the present high-water mark. The beaches are composed of shingles of quartz rock (that of the island), of about the size of cocoa-nuts, and they are precisely similar to those which constitute the present beaches on the Loch Tarbert side of Jura, where these raised beaches are well seen. Their aggregate breadth varies "according to the disposition of the ground: where the slope is precipitious, it may be a hundred yards; where gentle, as on the north side of the loch, three-quarters of a mile from the shore."* The beaches repose partly on bare rock, and partly on a compound of clay, sand, and angular pieces of quartz rock. Captain Vetch observed that caves are found at the same level on the north side of Loch Tarbert, at a considerable height above the sea, and as he had never seen caverns formed in the quartz rock of Isla, Jura, or Fair Island (Hebrides), except on the shore, he considers these to have been formed at the time when the relative levels of sea and land were such as to cut the line of the caves.

* Geological Transactions, 2nd series, vol. i.

CHAPTER XXV.

TEMPERATURE OF THE EARTH.-TEMPERATURE OF DIFFERENT DEPTHS IN SIBERIA.-TEMPERATURE FOUND IN ARTESIAN WELLS.-HEAT OF WATERS RISING THROUGH FAULTS AND OTHER FISSURES.-VARIABLE TEMPERATURE FROM UNEQUAL PERCOLATION OF WATER THROUGH ROCKS.-TEMPERATURE OF WATERS IN LIMESTONE DISTRICTS.

As the temperature of the earth may have an important bearing upon conclusions which an observer might feel disposed to form respecting the causes of certain phenomena which he may be investigating, it is desirable that he should carefully direct his attention to it, so that its full value, as a geological agent, may be duly appreciated. Mention has been above made (p. 206), of the heights above the level of the sea at which, with certain modifications, water remains in a solid state. Independently of the wellknown action of the sun on the surface of the earth, it is found that, after due allowance has been made for the temperature thus produced, there is another temperature, commencing at certain distances beneath that surface, the cause of which appears to require another explanation. Diurnal variations of temperature are considered not to extend, viewing the subject generally, to a greater depth than about three feet, and annual variations are inferred to cease at from 65 to 70 or 80 feet. Beneath depths not much differing from the latter, the temperature of rocks has been found to increase in mines, as also in the perforations into the ground commonly termed artesian wells. The rate of this increase of temperature has been found to vary, as might be expected, from certain local causes, such as the relative exposure of the mass of ground examined with respect to the form in which it may project into the atmosphere, should it be a mountain, its proximity to any particular source of heat, such as a volcanic region in activity, and the different circulation of water amid its parts, either among fissures or through beds of rocks of variable porosity.

We have seen (p. 291) that in the colder regions of the