Handbook of Paleontology 19 



accumulate on bottoms both in fresh-water ponds and 

 seas forming a diatomaceous ooze. The rock formed is 

 known as diatomaceous earth or tripolite, when pure, 

 (named from its occurrence at Tripoli in North Africa). 

 Skeletons of radiolarians and sponge spicules also gen- 

 erally occur in such deposits. 



Foraminifera secrete shells of carbonate of lime 

 which accumulate to such an extent that they form 

 oozes on the ocean bottoms, such as the Globigerina 

 ooze of today and Globigerina limestones (Tertiary) 

 of the past. The chalk cliffs of France and England 

 (Cretaceous) are mainly of foraminiferal origin, as 

 are also the nummulitic limestones (Tertiary) of 

 northern Africa and southern Europe and the Fusu- 

 lina limestones (Carboniferous) of western North 

 America and Europe. Some of the limestones of the 

 Gulf States and the West Indies are formed of species 

 of Foraminifera related to the Nummulites. Corals 

 with associated lime-secreting forms, such as the 

 hydrocorallines among animals and calcareous algae 

 among plants, are responsible for reefs and lime de- 

 posits today and in the past. Older limestones show 

 by certain structures that they are partly formed of 

 old reefs, partly of bedded lime deposits consisting of 

 a mixture of coral sands, shell fragments, etc. Some- 

 times these ancient reefs are very clearly defined; 

 again they may be so altered that they can be recog- 

 nized only through their general form and structures 

 in the surrounding rock. Fossil reefs are found in the 

 Silurian of Wisconsin and adjoining areas and in the 

 Middle Devonian of eastern United States. These 

 Devonian reefs were formed successively in the great 

 interior sea of that time along the coast of the land 



