Subsurface Laboratory Methods 97 



long, slender needles, may be branching stag-horn types, or may be broad 

 and thick, resembling the horn of a moose. 



The coralline algae secrete lime within and between the cell walls, 

 thus showing definite microstructure (fig. 42). The various genera are 

 separated on the basis of arrangement of cells in the tissue and the char- 

 acter and arrangement of the spore cases (conceptacles) . Individual 

 species within a genus are separated on the basis of cell and spore-case 

 dimensions.^ 



Coralline algae develop in all seas from the poles to the equator 

 and from tide level almost to the maximum depth of light penetration. 

 Certain genera and individual species have specialized to accommodate 

 certain ecologic environs. Coralline algae are recorded from rocks of 

 all geologic ages from the Ordovician to the present. They are important 

 after Late Cretaceous. Although they grow abundantly in most of the seas 

 of the world, they attain their greatest development in the tropics, par- 

 ticularly in and around the reefs. 



In the atolls of the Marshall Islands algae are very important as 

 builders of reef limestones; locally they form up to 80 percent of the 

 reef rock. In the fringing reefs of the Marianas, corals predominate, but 

 algae still play a very important part both as contributors to the limestone 

 end by acting as binding agents in the reefs. 



Coralline algae produce distinctive fossils, and, because of their 

 microstructure, they can be exactly identified in thin sections. Except in 

 limited areas, they have not yet been sufficiently studied to determine the 

 geologic range of the species. In those areas, however, where they have 

 been studied, most of the species appear to have restricted time ranges 

 and, hence, have possibilities for serving as guide fossils. They can give 

 considerable ecologic information. 



Dasycladaceae 



The Dasycladaceae are small, bushy plants belonging to the green 

 algae (fig. 41, nos. 1-4). They consist of a central stem with regularly 

 spaced whorls of primary branches radially arranged. These may bear 

 secondary and tertiary branches. Calcium carbonate is deposited around 

 the central stem and primary branches, forming a shell of variable thick- 

 ness. The type of fossil obtained and the amount of structure it exhibits 

 depends upon the degree of calcification. The fossils usually appear as 

 small rods or club-shaped fragments. A few are spherical; some are 

 disc- or umbrella-shaped. They commonly range in length from about 

 one-eighth of an inch to more than three-fourths of an inch, although a 

 few may grow much larger. Some have characteristic shapes and conse- 

 quently are easily recognized, whereas others need to be sectioned for 

 identification. Typically the structure consists of a mold of the central 

 stem from which whorls of tiny branches develop at regular intervals. 



* Lemoine, Mme. P., Algues calcaires jossiles de I'Algerie:.. Materiaux pour la Carte Geol. de 

 I'Algerie, 1^ ser., no. 9, 128 pp., 3 pi., 1939. 



