FOSSIL FORMS 267 



in colloidal materials or perhaps in calcareous muds. In the original 

 description Walcott suggested an affinity to the Cyanophyceae, but 

 as later workers could only distinguish a purely mineral structure 

 they suggested the idea of diffusion phenomena. Discoveries of 

 very comparable algal concretions and laminations in the Bahamas, 

 however, have made it extremely probable that these structures had 

 an algal origin. Some examples of these types are shown in fig. 168. 

 On the basis of Black's discoveries (1933) it may be suggested that 

 these structures were not necessarily formed by deposition but that 

 the algae collected and bound the sediment. 



•■A",~-.=t>..: 



D E f 



Fig. 168. Stromatolithi. A, Weedia. B, Collenia. C, D, Cryptozoon. E, 

 Archaeozoon. F, Gymnosolen. (After Hirmer.) 



Porostromata (e.g. Girvanella, Sphaerocodium). 



These forms, which are most abundant in the Carboniferous, 

 have a recognizable microscopical structure, the threads often being 

 arranged in a radiating fashion: they were probably formed in 

 much the same way as the algal water-biscuits now found in 

 South Australia. These range from tiny particles to thick bun-like 

 forms 20 cm. in diameter, whilst in them are to be found the tube- 

 like remains of living species of Gloeocapsa and Schizothnx, 

 Gloeothece and Gloeocapsa are also known to form oolitic granules 

 in the neighbourhood of Salt Lake City. The presence, however, of 

 pebbles, or the existence of a granular structure, does not necessarily 

 involve the presence of algae, and in some cases it is also possible 

 that the algae were merely included through chance. Pachytheca is 

 a genus from the Silurian and Devonian which possesses a medulla 



