38 AN INTRODUCTION TO THE STUDY OF FOSSILS 



Lithothamnion has been one of the principal reef builders since 

 the Cretaceous. 



c. Here also may probably be placed such early Paleozoic 

 calcareous masses as Cryptozoon (Fig. 9). 



Lime secretion. — It is in the securing of carbon from the 

 decomposition of CO2 for the building of their tissues that algae 

 secrete lime. Land plants derive the CO2 from the air. Aquatic 

 plants must secure it from the water. An excess of CO2 in the 

 water holds lime carbonate in solution, and when the CO2 is 

 abstracted from the water by the plants, the lime is of necessity 

 thrown down, and thus is deposited in or upon the tissues of 

 the plant which caused its precipitation. 



Algae as rock-builders. — Algae have been found to be in many 

 cases, at least, the most important lime contributors to the up- 

 building of the coral reefs, much more important than the corals 

 themselves. A boring in the atoll of Funafuti penetrating to 

 a depth of 11 14.5 feet showed that in the composition of this 

 typical " coral-reef " the order from most to least abundant 

 of the most common organisms is as follows : i . Lithotham- 

 nion ; 2. Halimeda; 3. Foraminifera, a lime-secreting order of 

 Protozoa; 4. Corals. All these occurred from top to bottom 

 of the boring. Thus in the formation of this atoll nuUipores 

 were found to be the most important agent. This is true to a 

 greater or less degree of many other " coral-reefs " of the Pacific 

 Ocean, also of the East Indian and West Indian waters and the 

 Mediterranean. Of these nullipores, Lithothamnion grows 

 abundantly at a depth of two to three hundred fathoms; it 

 also occurs in waters far from the tropics; off the coasts of 

 Spitzbergen and Nova Zembla it covers the bottom in water 

 of ten to twenty fathoms. The growth of nullipores may be 

 faster than that of corals since they often cover and smother liv- 

 ing colonies of the latter. Lime-secreting algae are thus seen to 

 be of vast geologic importance in the formation of limestones 

 since they grow so rapidly and at such various depths and 

 temperatures. But evidences as to the nature of the organisms 



