

I 









hi 



I 







i 





Ch XVII.] 



TRAVERTIN IN TIVOLI. 



407 



di JSTettuno. I have not attempted to express in this draw- 

 ing the innumerable thin layers of which these magnificent 

 spheroids are composed, but the lines given mark some of 

 the natural divisions into which they are separated by minute 

 variations in the size or colour of the laminae. The undula- 

 tions also are much smaller in proportion to the whole cir- 

 cumference than in the drawing. The beds (a a) are of hard 

 travertin and soft tufa; below them is a pisolite (6), the 

 globules being of different sizes : underneath this appears a 

 mass of concretionary travertin (c c), some of the spheroids 

 being of the above-mentioned extraordinary size. In some 

 places (as at d) there is a mass of amorphous limestone, or 

 tufa, surrounded by concentric layers. At the bottom is 



small 



are 



about the size and shape of beans, and some of them of 

 filberts, intermixed with some smaller oolitic grains. In the 

 tufaceous strata, wood is seen converted into a light tufa. 



There can be little doubt that the whole of this deposit 

 was formed in an extensive lake which existed at the close 

 of the period of volcanic activity by which the lavas and 



tuffs of the 



formed 



The external 



configuration of the country has since been greatly changed, 

 and the Anio now throws itself into a ravine excavated in 

 the ancient travertin. Its waters pwa risA to 



masses 

 from tl 



rock. I was shown, in 1828, in the upper part of the tra- 

 vertin, the hollow left by a cart-wheel, in which the outer 

 circle and the spokes had been decomposed, and the spaces 

 which they filled left void. It seemed to 



o me at the time lm 



mould, without arm 



imbedded 



drained; but Sir E. 



been washed down by a flood into the gorge in modern times, 



Murchison suggests that it may 



manner 



as the wooden beam of the church of St. Lucia was swept 

 down in 1826, and stuck fast in the Grotto of the Syren, 



remains 



in travertin.* 



* Murchison, Geol. Quart. Journ., 1850, vol. vi. p. 293. 



