10 piATOMAcr.i:. 



The principal fossil deposits of diatomacese hitherto discov- 

 ered contain marine species, and extend over considerable 

 tracts of the earth's surface. The most important stratum of 

 this character is considered to belong to the miocene tertiary, 

 and is found on the Atlantic side of North America, not far 

 from, and, in fact, in some places, reaching down to the coast. 

 It is known to extend from the Patuxent river, in Maryland, as 

 far south as the city of Petersburg, in Virginia. How much 

 beyond these two points it extends has not been ascertained, 

 but is found underlying the cities of Petersburg, Richmond and 

 Fredericksburg, in Virginia, and at many other points in that 

 State, as well as in Maryland. It is desirable to obtain speci- 

 mens from different points in this bed, as it varies in character, 

 and contained organisms with every few miles of surface, and 

 at different points in its depth. 



Strata of this kind vary greatly in appearance, as well as in 

 microscopic character. Therefore the following general direc- 

 tions will suffice to guide collectors in searching for and detect- 

 ing them. 



Gather all earths of light color, varying from a pure white, 

 through different shades of grey, cream, and fawn, to an iron- 

 rust tint. The texture is often friable, and then looks some- 

 what like clay, especially when it is wet; at other times it is of 

 a hard and stony character, though always more or less porous, 

 and, when soft, of little weight. A moderate magnifying 

 power shows it to be made up of the shells of diatomacese. 

 Collect enough to make up three or four pounds' weight, or, 

 say, a block six or eight inches square, and, if possible, from 

 the surface and at various depths, for the* reasons already 

 stated. Some of the localities of this material may be men- 

 tioned. In Virginia it has been procured in and near Peters- 

 burg and Richmond, at Shockhoe Hill and Church Hill, and at 

 Hollis Cliff; and in Maryland, at Lower Marlboro', Nottingham, 

 Piscataway, and Rappahannock Cliff. 



Besides the above mentioned, an extremely interesting stratum 

 of a similar character, but in general of harder texture, has 

 been found on the Pacific coast of North America, and extend- 

 ing at least from San Francisco to the lower border of Cali- 

 fornia, if not further, in both directions. This substance 



