708 



DYNAMICAL GEOLOGY. 





sketch below, from a photograph by W. H. Jackson. Travertine is 

 throughout concretionary, and in many parts cavernous, and com- 

 monly wholly unlike the even-grained material of ordinary limestone 

 strata, Leaves, stems, and nuts are often petrified by the calcareous 

 waters. 



The waters dripping into limestone caverns produce, by their calca- 

 reous depositions, the pendent stalactites of the roof of the cavern, and 

 the stalagmite of the floor. The stalagmite shows, in a cross frac 

 ture, the fact of its gradual deposition, by the bandings in its colors^ 

 The deposit from such waters sometimes has a soft chalky texture. 



Fig. 1108. 



Travertine deposits on Gardiner's River. 



Some sand and clay beds owe their consolidation to carbonate of lime 

 derived from the remains of shells present in them. 



2. Through Siliceous Waters. — Siliceous waters have done far the 

 larger part of the consolidation of sandstones, conglomerates and clay 

 beds. The silica has commonly been taken up from feldspars distrib- 

 uted throughout the rock itself, or from the siliceous relics of Diatoms 

 and Sponges present in it, by the heat and moisture penetrating it ; 

 and then consolidation has taken place as the temperature lowered. 

 Such solutions have filled fissures and cavities in rocks with quartz, 

 making quartz seams and veins. They have also been one means by 

 which mineral silicates have been formed in the process of meta- 

 morphism (p. 760). 



