ALGJE AS ROCK-BUILDING ORGANISMS. 17 



describes from the Upper Alpine Trias and places in the 

 family Codiaceee} It occurs in small spherical or oval ag- 

 gregations which were formerly regarded as concretionary 

 oolitic spherules. In the Raibler and St. Cassian beds 

 there occur bands of limestone to a large extent built up 

 of these Sphcurocodium spherules ; Rothpletz figures and 

 describes this alga as S. Bornemanni. 



According to Bornemann 2 a fossil genus of the Ri- 

 vulariacece has also been responsible for oolitic structure in 

 some cases ; he institutes the genus Zonatric kites for this 

 form, and gives the following definition of it : "A fossil 

 calcareous alga with radially arranged filaments, forming 

 hemispherical or kidney-shaped layers, growing on or en- 

 closing- other bodies. Parallel or concentric zones are seen 

 in cross section, formed by the periodic growth of the alga, 

 the older layers serving as foundations on which the 

 younger filaments grow in radially arranged groups." 



In an appendix to Bornemann's paper the question of 

 oolitic structure is dealt with, and descriptions are given 

 of microscopic preparations of oolitic rocks showing 

 the nature of the algal elements in the individual 

 grains. 



In his paper on rock-building algae in the Swiss Alps, 

 Friih 3 calls attention to the association of Zonatrichites 

 with oolitic structure. 



As another example of recent algae as oolite-builders, 

 reference should be made to Weed's description of the 

 travertine deposits at the Mammoth hot springs in the 

 Yellowstone Park. 4 



Enough evidence has been adduced, without citing 

 further examples, to show that there is reason to believe 

 that algae have a much greater claim to the attention of 

 geologists as possible agents of rock construction than has 

 generally been admitted. 



1 Rothpletz (1), pi. xv., figs. 5 and 6, etc. 



2 Bornemann (2), pis. v. and vi. 

 8 Fruh. 



4 Weed, p. 643. 



2 



