D. B. WARD, M. D. 75 



bacteria themselves, while the diatom spores retain their 

 vitality when dry, acondition which would be fatal to the 

 perfect plant. Pools of water, ditches, etc., dry up and 

 the diatoms are killed, but the spores remain and some 

 of them are blown about by the wind and find some wet 

 place to develop, and when it rains those remaining in 

 the pool spring into life and repopulate the water. This 

 accounts for the wide diffusion of species. 



As the valves of the diatoms are practically indestruc- 

 tible, when the plants die the empty shells settle to the 

 bed of the ocean or lake where they grew, and thus in the 

 course of time they have in various parts of the world 

 formed vast deposits in which the shells remain as per- 

 fect ^s when they were living. Many of these fossil 

 species have long since become extinct. The city of 

 Richmond, Virginia, is built over such a deposit, which 

 was laid down during the tertiary period, and this de- 

 posit is in places forty feet thick. Some specimens of 

 this earth consist of diatoms and nothing else, others 

 have a good deal of fine sand mixed with them, but in all 

 the diatoms are very abundant. 



Three or four years ago Mr. Lewis VVoolman, a geolo- 

 gist of Philadelphia, in examining the material brought 

 up by the drill of an artesian well at Atlantic City, N. J., 

 found a large number of diatoms at a depth of 406 feet 

 from the surface. These are in a brown miocene clay, and 

 at 650 feet another rich diatomaceous clay was reached 

 and still another at 625. The intervening strata con- 

 tained diatoms in smaller quantities at all depths. Sub- 

 sequently other wells pierced these strata at different 

 depths in other places. At Weymouth the upper stra- 

 tum is only forty feet below ground, and at Shiloh there 

 is an outcrop. It would seem therefore that a large por- 

 tion of the State of New Jersey has an enormous bed of 

 diatoms beneath it some 300 feet thick. In this miocene 

 clay have been found many new species and other forms 



13 



