344 



W. H. Twenhofel — Pre-Cambrian and 



algae were probably related to light in their distribution 

 just as they are to-day, and this factor would have con- 

 fined them to the shallows about the margins of the lands 

 which they would have reefed just as the corals and algae 

 reef tropical lands at the present time. Places where 

 clastic sediments were being deposited in the Huronian 



Fig. 1. 



Fig. 1. Collenia Tcona n. sp. Cross section of a single colony, Kona 

 dolomite, Marquette, Mich. Photograph by Dr. W. O. Hotchkiss. 



sea or were being transported along the shore would have 

 been free from algaa. Their places of abundant growth 

 would have been found in clear and shallow waters. This 

 fact of distribution probably explains their general 

 absence from this dolomite formation. It is quite likely, 

 however, that some of the dolomite which shows no evi- 

 dence of algal origin may have so originated; but have 

 lost all traces of its origin in the process of recrystalliza- 

 tion. 



