TUFA TOWERS. 



209 



interior of the basin A cross-section of one of these cohuims is shown in 

 the following diag-nim. The central portion (a) is of comi)act lithoid tufa, 

 which usually exhibits a concentric or tubular structure, and is very fre- 

 (jaentl}' from G to 10 feet in diameter; surrounding this core is a layei- of 

 thiiiolite crystals, forming the concentric band (b), which is commonly from 

 2 to 6 feet thick ; outside of this layer is a coating of dendritic tufa (c), 

 of somewhat greater thickness than the thinolite layer, which sheathes the 

 outside of the tower and arches over the low dome forming its summit. 



Fig. 29. — Vel'ticul aud lionzmital .s. ctiuus of a tufa tower. 



ot only are these isolated towers composed of. distinct layers of the 

 three varieties of tufa we have mentioned, but each of these main divisions 

 is itself banded. The cross-section of some of the tufa towers shows that 

 tlie inner core of lithoid tufa is composed of as many as fifteen or twenty 

 distinct envelopes ; at the center and near the outer margin, some of these 

 l)ands have a dendritic structure. Sections of the middle or thinolite mem- 

 ber reveal that it also is composed of a large number of concentric bands, 

 some formed of large, and some of small crystals ; near the outer portion 

 of this deposit thin layers of thinolite alternate with narrow bands of den- 

 dritic tufa. The outer sheathing of dendritic tufa is also composed of many 

 lavers. Each of these concentric circles seen in the cross-section of a tufa 

 tower, like the annual rings of an exogenous tree, is a section of a cylinder 

 that lias been formed about the previous one. It is evident that each con- 

 centric band records a change of greater or less importance in the character 

 MoN. XI 14 



