NATURAL HISTORY OF THE DIATOMACE^E. 479 



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

 scopic character. Therefore the following general directions will suffice 

 to guide collectors in searching for and detecting 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 somewhat 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 diatomaceae. 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 mentioned. In Virginia it has 

 been procured in and near Petersburg 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 extending at least from San 

 Francisco to the lower border of California, if not farther, in both direc 

 tions. This substance makes up the major part of the rocks of the coast 

 range of mountains, and has been named the bituminous shales. It was 

 first detected at Monterey, and is known to microscopists in England as 

 &quot; Monterey stone,&quot; but it has since been traced and brought from various 

 points. Santa Cruz, San Pedro, and San Diego have yielded excellent 

 specimens containing many beautiful forms of diatomaceae. It is usually 

 light fawn-colored, and distinctly stratified. Large fossil shells are found 

 in it ; and associated with and in, if not derived from it, is the bitumen of 

 California. At Baldjik, near Varna in Bulgaria, on the Black sea, is a 

 stratum of stony character, having shells and bones dispersed through it. 

 The diatomaceae found in it are apparently of brackish-water origin, and 

 this is the only stratum of this kind that is known. But very little of 

 this material has found its way into the hands of naturalists. On the 

 island of Jutland, in Denmark, is found a polishing slate which is rich in 

 diatomaceous forms not found anywhere else. This, also, is rare among 

 naturalists, and a good supply of it is very desirable. At Oran in 



