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Show 214 FERRUOINOUS SPlHNGS· country. The solution of tho silex is supposed to be promoted by the presence of some mineral alkal~. In some of t~e.thermal waters of Iceland a vesicular rock IS formed, contammg portions of vegetables, more or less completely silicified. Amongst the various products also of springs in this island, is that admixture of clay and silica, called tripoli. It has been found, by recent analysis, that several of the thermal waters of Ischia are impregnated with a certain proportion of silica. Some of the hot vapours of that island are above the temperature of boiling water; and many fissures, near ~onte Vico, through which the hot steam passes, are coated With a siliceous incrustation, first noticed by Dr. 'fhompsem under the name of fiorite. In some places where silicification is in progress, the sout·ces from whence the mineral matter is derived are as yet unknown. Thus the Danube has converted the external part of the piles of Trajan's bridge into silex; and the Irawadi, in Ava, has been supposed, ever since the time of the Jesuit Padre Duchatz, to have the same petrifying power, as has also Lough Neagh, in Ireland. Modern researches, however, in the Burman empire, have not confirmed, but have rather thrown doubt upon the 1apidifying property of the Ava river*. The constant flow of mineral waters, even when charged with a small proportion of silica, as those of Ischia, may supply certain species of corals and sponges with matter for their siliceous secretions; but when in a volcanic archipelago, or a region of submarine volcanos, there are springs so saturated with silica, as those of Iceland and the Azores, we may expect beds of chert or ]ayers and nodules of silex, to be spread out far and wide over the bed of the sea, and interstratified with shelly and calcareous deposits, which may be forming there, or with matter derived from the wasting cliffs or volcanic ejections. Fer·ruginous Springs.-The waters of almost all springs contain some iron in solution ; and ·it is a fact familiar to all, that many of them are so copiously impregnat~d with this metal, as to stain the rocks or herbage through whiCh they pass, and to bind together sand and gravel into solid masses, W c may * Dr. Buckland, Geol. Trans., second series, vol. ii., pa1·t 3, p. 384. BRINE SPRINGS. 215 naturally, .therefore, conclude that this iron, which is constantly conveyed mto lakes and seas from the interior of the earth and not returned again to the land by evaporation in the atmospheric waters, must act ~s a col?uring and cementing principle in the s~1baqueous deposits now In progress. When we find, therefore, that so man~ sandstones and other rocks in the sedimentary strata of ancient lakes and seas are bound togethet· or coloured by iron, it presents us with a striking point of analogy between the state of things at very different epochs. In the older formations we meet with great abundance of carbonate and sulphate of iron; and in chalybeate waters at present, this metal is most frequently in the state of a carbonate, as in those of Tunbridge, for example. Sulphuric acid, however is often the solvent, which is in many cases derived from the de~omposition of pyrites. Brine Springs.-So great is the quantity of muriate of soda in some springs, that they yield one-fourth of their weiO'ht in salt •. Tl~ey ar~ rarel_y, howevet; so saturated, and ge~erally contam~ mternnxed With salt, carbonate and sulphate of lime, magne.sia, and oth~r min:ral ingredients. The brine springs of Cheshire are the nchest In our country; those of Barton and ~orthwich being almost fully saturated. These brine springs rise up through strata of sandstone and red marl, which contain large ~eds ~f ro~k-salt. The origi~ of the brine, therefore, may be del'lved m this and many other mstances from beds of fossil salt; b.ut as muriate of soda is one of the products of volcanic emanatiOns and of springs in volcanic regions, the original source of salt may be as deep seated as that of lava. . The w~t~rs of the ~ead Sea contain scarcely anything, except muriatic salts, wluch lends countenance, observes Dr. Daubeny' t~ the volcanic origin of the surrounding country, these sul~.s be~ng. f~equent yroducts of volcanic eruptions. Many ~pnngs In SICily contam mur·iate of soda, and the "fiume salso ,, 111 par t•I cu 1a r, I.s I. mpregnated with so large a quantity that cattl' e refuse to drink of it. If rivers or springs, thus im'pregnated, cute~ a lake or estuary, it is evident that they may give rise to partial precipitates of salt. A A hot spring, rising through granite, at Saint Nectaire in ~ ergne, may be mentioned as one of many, containing' a |