474 PRINCIPLES OF STRATIGRAPHY 



so that the aragonite is changed to iron carbonate which subse- 

 quently alters to iron oxide as outlined by Sorby and others. 



Sph^rocodium and Girvanella Deposits. In the Palaeozoic 

 and later limestones of many coimtries are found rounded masses of 

 lime ranging in size up to several centimeters and referred to the 

 genera Sphasrocodium and Girvanella. These often occur in suf- 

 ficient abundance to constitute a large part of the rock mass, as in 

 the case of the Sphserocodium beds of the Siluric of Gotland. 

 Sph?erocodium forms rounded masses generally coating foreign 

 bodies and consists of a simple network of unicellular threads, which 

 result in the formation of successive shells of calcareous tissue. 

 The genus has been found abundantly represented in the Siluric of 

 Sweden and in the Triassic of the Alpine region. 



Girvanella consists of irregularly twisted, tubes and forms ir- 

 regular knobby or rod-like masses, also, as a rule, adhering to 

 foreign bodies. It has been found in the Ordovicic of England and 

 America and in the Carbonic limestone (Mississippic), the Superior 

 oolite and the Coralline oolite of England. 



Other algous types of the Palaeozoic, with similar mode of 

 growth, are Siphonema, found in the Ordovicic, and Zonotrichites 

 from the Rhsetic. (Rothpletz-35 ; 37.) Algae have also been de- 

 scribed from the Trenton limestone of New York. (Ruede- 

 mann-38. ) 



Fossil Nullipores. The fossil nullipores are well represented 

 in older geological formations. As already noted, the Triassic reefs 

 of the Tyrol are regarded by many as chiefly of nuUipore origin. 

 The leading alga is a coralline. Diplopora, which occurs largely as 

 dissociated joints of the branches. These algae are closely related 

 to the living Cymapolia, which, like Halimeda, is a jointed siphona- 

 ceous member of the Chlorophyceae or green algae. The red algae 

 (Rhodophyceae) are also well represented from the Jura to the 

 Pliocenic, some fifteen or more species being known in a fossil state. 

 Sometimes they formed extensive encrustations on the shores and 

 islands of ancient seas, as in the case of the Miocenic Leythakalk 

 of the Vienna basin, formed chiefly by Lithothamnion ramossimum 

 Reuss. This nullipore forms bundles and mats of numerous short, 

 wart-like or club-shaped branches from 2 to 5 mm. in diameter. As 

 the lime is in the form of calcite, it is commonly well preserved to- 

 gether with other calcite structures, while associated fossils of 

 aragonite are for the most part dissolved away and the lime re- 

 deposited as calcite among the algous masses. (See Mojsisovics- 

 29 -.498. ) 



Fossil Chara. Fossil stems and fruits of Chara are responsible 



