34 



AN INTRODUCTION TO THE STUDY OF FOSSILS 



forms responsible for the hot springs deposits in the Yellow- 

 stone Park. The origin of many of the oolites of the Jurassic 

 of England, — Superio'r, Great oolite, etc., — has been referred 

 to these algae as has likewise that of the oolites of the Bunter 

 sandstone (Triassic) of Germany. 



SUB-DIVISION C, DIATOME.E 



The diatoms are a group of thallophytes with possible kin- 

 ship with the algae. They are microscopic, one- celled plants, 

 inclosed in two valves of which one overlaps the other. This 

 cell wall is impregnated with silica and hence forms a resistant 

 skeleton (Fig. 7). Reproduction is either asexual, through 



division of the cells, or sexual, through 

 union of two cells to form a new 

 individual. 



Diatoms occur in both fresh and salt 

 water as well as in damp soil. In the 

 ocean they form a large part of the 

 plankton, — the drifting mass of organ- 

 isms at the surface of the water, and 

 furnish the food of many marine ani- 

 mals. So great is their abundance that 

 their skeletons, cast off at death or in 

 the process of reproduction, form on 

 the ocean bottom or on the bottom of 

 ponds or marshes deposits of siliceous 

 earth, — the diatomaceous ooze. Such 

 deposits are known from the Jurassic to 

 the present, especially in the Miocene 

 of California and of the Atlantic coastal plain. One such 

 deposit between the Santa Yuez and Los Alamos Valleys, Cali- 

 fornia, reaches a thickness of forty-seven hundred feet. 



Fig. 7. — Examples of the 

 silica-secreting algae, dia- 

 toms, abundant in the 

 ocean covering eastern 

 Maryland during the 

 Miocene (Calvert) times. 

 A , Coscinodiscus lineatus 

 Ehrenberg, a circular 

 species. B, Sceptroneis 

 caduceus Ehrenberg, a 

 lanceolate species. Each 

 X 170. (From Boyer.) 



