88 



AGRICULTURAL BACTERIOLOGY 



was deposited on the floor of the sea, and in time limestone 

 deposits were produced. By some movement of the earth's 

 crust the floor of the sea was lifted above the water-level and 



Fig. 20. Etching of Marble by Plant Roots 



The picture on the left shows the solvent action of plant roots on a marble 



slab in the absence of bacteria; the one on the right the action in the presence 



of bacteria. The latter have formed acids from the material given off by the 



plant and have increased the solvent action on the calcium carbonate 



After Fred. 



subjected to weathering. These deposits were also ground 

 by the action of the ice in the great glaciers that swept over a 

 large part of the country, or they were disintegrated by the 

 weather, so that a great mass of limestone found its way into 

 the soil. 



The carbon-dioxide dissolved in the soil water increases 

 the solvent action of the water on the calcium carbonate, 



