470 PRINCIPLES OF STRATIGRAPHY 



MgCOg, 5.5% ; CaSO^ and Si02, 0.5% ; organic matter 3.8%. This 

 alga forms extensive limestone deposits within the lagoons of coral 

 atolls, as in the case of Funafuti, and in the Red Sea. 



Order PhcBophycecc or Broivn Algce. 



This division, which contains the great algae (Laminaria, Ma- 

 crocystis), the floating Sargassum, and the attached rock weed or 

 Fucus, has no important lime-secreting members, but the diatoms, 

 important on account of their siliceous frustules, belong here. Their 

 general character and importance have already been discussed in 

 the preceding chapter. 



Order Rhodophycecc or Floridccc — Red Alga. 



This division contains several very important types which act 

 in part as rock-builders. These include the genera Corallina, Jania, 

 Melobesia, Lithothamnion, and Lithophyllum and they are dis- 

 tributed from the equator to the arctic regions. The living plant 

 often contains as much as 85% CaCOg and many of the more mas- 

 sive ones (Lithothamnion, etc.) build heavy barriers on the outer 

 zone of coral reefs, where wave activity is most pronounced. Litho- 

 thamnion fasciatmn occurs in the Arctic Sea on the coast of Nor- 

 way at a depth of 70 m., on the Russo-Lapland coast from 10 

 to 55 meters, at Spitzbergen, 18-36 m., and at Nova Zembla in 45 

 meters. L. polyuwrphum and Corallina officinalis are found on 

 the Great Banks in 22 m. (W'alther). L. raceiiius forms masses of 

 the size of a fist in the Gulf of Naples on the Secca di Gajola in 

 25 meters depth, while the smaller L. raniolosum abounds in 45 to 

 60 meters on the Secca di Benda Palummo, or pigeon bank. (Wal- 

 ther-50 :i>i'9 ; 52:.//.) 



In the Caribbean Sea nullipores occur down to depths of 284 m. 

 (Agassiz-2 :/^7) and in the Gulf of Naples Lithothamnion, Coral- 

 lina and Lithophyllum occur at depths of 30 to 65 m. ( Wal- 

 ther-50 ri'i'p. ) The name "Nullipore" is generally applied to all 

 these lime-secreting algae. It is derived from the old Lamarckian 

 genus Nullipora. which included four species of calcareous organ- 

 isms, all probably belonging to the family Corallinaceae of the red 

 algae. 



The name Lithothamnion was generally applied to the unseg- 

 mented coralline algcC, but has more recently been restricted, while 

 other generic groups — Lithophyllum, Goniolithon, Phymatolithon, 

 etc. — have been separated from the original group. 



