692 



DYNAMICAL GEOLOGY. 



alternately wet and dry, become coated with a deposit of carbonate 

 of lime from the waters ; and finally all are united into a solid mass. 

 Sands and pebbles of other kinds are treated in the same way ; and, 

 on shores bordered by coral reefs, the pebbles of basalt, and other 

 kinds, often have a milky exterior, from a film of carbonate of lime. 



The calcareous mud and sand of the reef under water become 

 solidified apparently without other means than the carbonated sea- 

 waters. 



Beds of limestone are sometimes made by depositions from calca- 

 reous waters, though small beds, compared with those of organic origin. 

 The travertine of Tivoli, near Rome, is a large deposit along the Anio 

 (p. 75), whose waters are there strongly calcareous. On the banks 

 of Gardiner's River, in the region of the Yellowstone Park, in the 

 Rocky Mountains, thick limestone deposits have been made, from 

 the waters of numerous and large hot springs and geysers, as well 

 illustrated and described in the Reports of Dr. Hayden. The cal- 

 careous waters, in descending the slopes of the hills, have made a 

 series of parapets at different levels, inclosing basins, over which the 

 water drips or plunges on its way to the bottom, as illustrated in the 



Fig. 1107. 



Travertine deposits on Gardiner's River. 



sketch above, 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 



