116 DYNAMICAL GEOLOGY. 



table, some of animal origin ; some fresh water, some ma- 

 rine ; some composed of silica, some of lime carbonate. 

 The two most important kinds are silicious fresh-water 

 deposits of vegetable origin, and lime carbonate, deep-sea 

 deposits of animal origin. 



Fresh- Water Deposits. It is well known that still 

 waters swarm with microscopic unicelled plants. Most of 



FIG. 63. A, diatoma vulgare : a, side view of frustule; ft, frustule dividing itself. 

 B, grammatophora serpentina : a, front and side view ; 6, front and end view of 

 dividing frustule. 



these have no shells and leave no deposit. But one kind 

 the diatoms form shells of silica. Generation after 

 generation of these leave their shells until deposits of great 

 thickness and extent are formed. In any clear and 

 sluggish stream, if we examine with the microscope the 

 slime on the stones at the bottom, we shall find living 

 diatoms. These are carried by freshets into ponds, lakes, 

 or seas into which the streams empty. Usually the mud 

 carried with them is so abundant that they will not be 

 detectable in the deposit thus formed. But in large, 

 deep, clear lakes, like Lake Tahoe, beyond the reach of 

 sedimentary deposit, the deep bottom-ooze is found to be 

 composed wholly of the accumulated shells of diatoms. 

 Also in the hot springs of California and in the pools 

 formed by the accumulation of these waters, diatoms are 



