478 PHYSICAL GEOGRAPHY. 



PART SEVENTH. 



Directions for Collecting, Preserving, axd Transporting 

 Specimens of Diatomace.-f. 



The diatomacea? constitute a group of organisms of so much interest 

 to the student of natural history, that it is desirable that specimens 

 should be collected in various parts of the world. That such collections 

 may be of value, it is necessary that they should be made in a proper 

 manner; and for the purpose of facilitating the making of such collections 

 these directions have been drawn up. The directions given should be 

 closely followed, as the methods described have been found, after consid- 

 erable trial, to be those yielding the most satisfactory results. As the 

 fossil deposits containing the remains of diatomacccc arc most readily 

 recognized, gathered, and forwarded, they will be first described. 



Fossil Deposits. Included under this head must be considered the 

 enormous sub-plutonic strata found on the Pacific coast of North Amer- 

 ica, so that the fossil deposits of diatomacenc may be said to contain both 

 fresh-water and marine species, though never in a mixed state. In some 

 cases the particular species present indicate the character of the piece of 

 water in which the deposit has accumulated, different forms, or groups 

 of forms, appearing in bays, ponds, lakes, marshes, springs, and rivers, and 

 at various points of elevation above the surface of the sea. 



The principal fossil deposits of diatomacerc hitherto discovered 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 clown to 

 the coast. It is known to extend from the Patuxcnt 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 under- 

 lying 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 specimens 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. 



