LIME PRECIPITATES OF RIVERS 



341 



Older Deposits of This Typr. What appears to be a deposit 

 closely related to the last type is found in the upper Permic Mag- 

 nesian Limestone series of Durham, England. Here many of the 

 beds are composed of more or less spherical masses of dolomite 

 having a radiating structure and varying greatly in size and form ; 

 pisolitic, botryoidal, reniform and large globular masses being com- 

 mon. 



Limestone Deposits from Rivers. Throughout the limestone 

 region of torrid America, especially on the margins of the bolson 

 plains of Mexico, a deposit of limestone is forming as a white 

 superficial crust, sometimes comparatively free from foreign ma- 

 terial, at others forming the cement of conglomerates. This con- 



FiG. 66. Spheroids of dendritic tufa. Basin of Lake Lahontan. 

 (After Russell.) 



centrate of lime is called tepetate, and consists of material dissolved 

 from the limestone surfaces, and transported in solution by the tor- 

 rential rivers. 



The Catinga limestone of Bahia, Brazil ( Branner-6 : ipj), is a 

 good example of a river-deposited limestone. The material is de- 

 rived from the older Salitre (Jurassic) or other limestones, and the 

 deposition probably began as far back as the Tertiary, and still con- 

 tinues. The attitude of the beds is generally horizontal, and their 

 thickness ranges up to nearly 100 feet. 



The deposition of this rock is due to the partial evaporation of 

 the lime-saturated stream waters as they leave the limestone regions, 

 and to the liberation of the carbon dioxide during the warming 

 under the semitropic sun. The deposition of lime usually begins at 



