TUFA. 



matter in folution, or mechanically fufpended. Tufa is alfo 

 formed by the concretion of loofe volcanic duft or cinders 

 cemented by water ; and alfo by the confolidation of mud 

 thrown out of volcanoes. The difintegration and fubfe- 

 quent confoHdation of bafaltic rocks forms a third kind of 

 tufa, which the German geologifts call trap-tujf. 



Calcareous tufa is formed in large quantities in many 

 lime-flonfr diftrifts. The waters which iflue from calcareous 

 rocks rife to the furface impregnated with a confiderable 

 quantity of carbonate of lime. On expofure to the air and 

 fight, they depofit their contents on whatever fubllance is 

 prefented to them, and in the beds of rivulets form folid 

 incruftations, fometimes of great extent, and many yards in 

 thicknefs. The ttone thus depofited is always more or 

 lefs porous or veficular, and contains portions of plants and 

 frefh-water (hells which it has enveloped. This ilone 

 hardens by expofure to the air, and is frequently employed 

 in architefture. It is not a little remarkable, that St. 

 Peter's church, and fome of the grandeft works of ancient 

 and modern archite&urc at Rome, are formed of a fpecies of 

 calcareous tufa, called by the Italians iravertino. In the 

 ricinity of Stroudwater, in Gloucefterfhire, many of the 

 fprings that ifTue from the feet of the Cotfwold hills make 

 large depofitions of calcareous tufa. At one fituation, called 

 the Rock-mill, a bed of this ftone, more than tliirty feet in 

 thicknefs, has been worked formerly for building, as ap- 

 pears from fome of the oldell edifices in the neighbourhood, 

 which proves the hardnefs and durability of this ftone. In 

 Derby fhire, and all the calcareous diftrifts in England, beds 

 of tufa are formed by fimilar depofitions. Beds of tufa 

 are very common among the mountains which terminate the 

 high vallies of the Alps, and there are promontories of cal- 

 careous tufa in the vallies of Switzerland. It was generally 

 fuppofed that the particles of lime-ftone were held in folution 

 by carbonic acid in the water, and that the evaporation of 

 this acid, after expofure to the air, occafioned the depofition 

 of the calcareous particles. Dr. Kidd, of Oxford, has 

 made fome experiments on the folvent power of fimple 

 water on carbonate of lime, which he finds to be much 

 greater than was generally fuppofed. If, fays he, half an 

 ounce of diililled water be agitated for a fhort time in con- 

 taft with pulverized carbonate of hme, it wiU be found, on 

 the addition of oxalate of ammonia, that the water will be 

 rendered turbid. It appears, therefore, that when a fpring, 

 charged with as much calcareous matter as it is capable of 

 holding in folution, has iffued from beneath the earth, the 

 requifite quantity has been diminifhed by evaporation, the 

 particles of calcareous matter are liberated and depofited. 



Calcareous tufa forms fo rapidly in fome fituations, as 

 entirely to clofe up the paffage in canals and aqueduAs 

 through which calcareous waters flow. Many hot fprings 

 depofit tufa in abundance : the heat appears to aflift the 

 folvent power of water, and to accelerate the precipitation of 

 its contents by a more rapid evaporation from the furface. 



The travertine, or travertine, is the moft remarkable of all 

 the calcareous tufas, as it has been employed for the con- 

 ftruftion of fome of the proudeft monuments of architeftural 

 genius in ancient and modern times. Of this ftone, 

 Breiflak, an Italiau mineralogift, gives the following in- 

 terefting defcription. 



" The Anio, or Tiverone, which defcends from the Apen- 

 nines of Vifcovaro and Subiaco to the eaft of Rome, crofTes 

 Tivoli before reaching the plain where it unites with the 

 Tiber. All the land through which the Anio palTes in 

 Tivoli, whether near the great cafcade or the fmaller ones, is 

 tilled with mafTes of a calcareous ftone, produced by the de- 



pofition of its waters. Sometimes a piece of ru(h or reed, or 

 other vegetable matter, is the firft point to which the calca- 

 reous earth begins to attach itfelf. It generally depofes in 

 concentric layers, and has the hardnefs and fibrous tiflue of 

 alabafter. Thefe layers are neverthelefs feparated by a 

 bed of calcareous earth, friable, yellowifh, and very fine. 

 At the foot of the mountain of Tivoli, where the Anio 

 enters the plain which extends to Rome, are the quarries of 

 travertine. This calcareous rock is depofed in horizontal 

 beds : its colour is yellowi(h-white, its grain earthy, frac- 

 ture uneven, and its hardnefs far furpaffes that of thofe cal- 

 careous mafies produced by the Anio in the neighbourhood 

 of Tivoli. Cavities, where the calcareous fubftance has 

 affumed a fparry grain and ftalaftitic form, are common in 

 travertine. Sometimes thefe cavities have been fince filled 

 by a calcareous ftalaftite, whiter, of a finer grain, and harder. 

 This is the origin of thofe white fpots, the regularity of 

 which has caufed them to be miftaken for marine bodies en- 

 veloped in its pafte. Travertine contains no remains of 

 marine fubftances, but fometimes it affords fragments of 

 vegetables. It is not doubted but travertine owes its 

 origin to the depofitions of the Anio ; depofitions which in 

 the plain may have formed a more folid and compaA rock : 

 becaufe its current was lefs rapid, and perhaps its waters 

 more ftagnant in feveral places. Not far from the quarry 

 of travertine is the Solfatara, fo called on account of the 

 great heat of its waters, which abound in ftilphuric hy- 

 drogen gas, and form a confiderable fediment of calcareous 

 matter. A cardinal d'Efte caufed the canal to be dug 

 which conveys the waters of the lake to the Anio. The 

 calcareous depofitions are there fo abundant, that if every- 

 three years it was not cleaned out, it would be clofed up, 

 notwithftanding its depth and breadth. The water which 

 runs in the canal, on meeting with the bits of ruih or other 

 bodies, covers them with a white calcareous cruft, two or 

 three lines in thicknefs. Thefe incruftations are known by 

 the name of comfits of T'rvoll. Before this paflage was 

 opened, the overflowings to which the lake is fubjeft were 

 often fo confiderable, that the water fpread over the neigh- 

 bouring grounds, and formed on their furface a ftony 

 cruft. The waters of the lake fo charged with calcareous 

 earth, uniting with thofe of the Anio in the floods which 

 their union muft produce, have themfelves contributed to 

 the formation of travertine. I do not think that the Anio 

 alone would have been capable of forming the quantity 

 which is found in that rock. 



" Independent of the immenfe quarries worked by the 

 ancients, there are befides others of fuch vaft extent, that 

 they may fupply the demands of many ages. The lake of 

 Solfatara feems to have greatly affifted in the formation of 

 this rock. Its water being charged with gaz, explains tlie 

 great number of hollows which travertine prefents. It 

 proves, that when the rock hardened, a gaz has at the fame 

 time efcaped in feveral places, which has prevented the ap- 

 proximation of its parts, which were ftill foft. As often 

 as the interior of a mafs of rocks prefents cavities without 

 any indication of foreign fubftances which might have op- 

 pofed the union of its parts, I conceive their origin may 

 be attributed to the elcape of gaz at the moment when the 

 fubftance was paffing from a ftate of foftnefs to folidity, by 

 cooling or drying. From what I have juft ftiewn, it follows 

 that the travertine or rock of Tiber or of Tivoli is a carbonate 

 of lime, formed by the depofitions of the Anio and the Solfa- 

 tara of TivoH. The Roman artifts give the name of traver- 

 tine only to the ftone taken from the quarry fituated at the 

 foot of the mountain of TivoU. The lithologifts, lefs Haves 

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