298 SCIENCE IN SHORT CHAPTERS. 



solvent is precipitated, and forms a crust on whatever is 

 covered by the water. 



When I visited this lake in the month of February it was 

 surrounded by a chevaux defrise of an extraordinary char- 

 acter; thousands of tribes of about half an inch to one inch 

 in diameter outside, with calcareous walls about one eighth 

 of an inch in thickness. These were standing up from two 

 to three feet high, and so close together that we had to 

 break our way through the dense palisade they formed in 

 order to reach the margin of the lake. After some consider- 

 ation and inquiry, their origin was discovered. They are 

 the encrusted remains of bullrusb.es that had flourished in 

 the summer and died down since. During the time of their 

 growth the water had risen, and thus they became coated 

 Avith a crust of compact travertine. This deposition takes 

 place so rapidly that a piece of lace left in the lake for a few 

 hours comes out quite stiff, every thread being coated with 

 limestone. Such specimens, and twigs similarly covered, 

 are sold to tourists or prepared by them if they have time 

 to stop. Sir Humphry Davy drove a stick into the bottom 

 of the lake and left it standing upright in the water from 

 May to the following April, and then had some difficulty 

 in breaking with a sharp pointed hammer the crust formed 

 round the stick. This crust was several inches in thickness. 

 That which I saw round the ex-bullrushes may have all been 

 formed in a few days or weeks. The rivulet that flows from 

 the lake deposits travertine throughout its course, and 

 when it overflows leaves every blade of grass that it covers 

 encrusted with this limestone. 



Near to the Lake of Tartarus is the Solfatara lake which 

 contains similar calcareous water, but strongly impregnated 

 with sulphureted hydrogen; it consequently deposits a 

 mixture of carbonate and sulphide of calcium, a sort of 

 porous tufa, some of it so porous that it floats. like a stony 

 scum, forming what the cicerone call "floating islands." 

 Lycll, in his ''Principles of Geology," confounds these 

 lakes, and describes Tartarus under the name of Solfatara. 



The travertine used as a building stone is chiefly derived 

 from the quarries of Ponte Lucano, and is the deposit that 

 was formed on the bed of a lake like that of Tartarus. The 



