Carboniferous Algal Deposits. 341 



These facts relating to algal deposits are of immense 

 importance in connection with the formation of limestone 

 and they become of greater significance when it is 

 remembered that many marine algae thrive almost as well 

 in cold waters as in warm and that in some cold waters 

 their deposits constitute a large percentage of the sedi- 

 ments. 



It would seem that in the Pre-Cambrian periods the 

 algae would have been likely to have had a greater abso- 

 lute importance than in later times. Before the advent 

 of the manifold variety of bottom animal life, they had 

 the entire submerged area at their disposal where they 

 dwelt undisturbed by competitors or enemies. When the 

 bottom animals appeared in abundance the algae fur- 

 nished food for many of them and had to compete with 

 them for space and this must have brought about some 

 decrease in their absolute and certainly in their relative 

 importance. 



In this paper are described the occurrence of algal 

 deposits in the Lower Huronian of Michigan and in the 

 Pennsylvanian and Permian of Kansas and Oklahoma. 

 For the opportunity of studying the field distribution of 

 the Kansas and Oklahoma occurrences the writer is 

 indebted to Mr. B. E. LaDow of Fredonia, Kansas; for 

 the opportunity of seeing the Huronian occurrence he is 

 indebted to Doctor C. K. Leith. 



Method of Origin of Algal Deposits of the Type Considered. 



The algal deposits considered in this paper are of the 

 incrusting or laminated type and one of them is quite 

 similar to the " water biscuits' ' of modern lakes. These 

 are thought to develop as a consequence of the absorption 

 of the carbon dioxide in the water and the release of 

 oxygen by the plant to the water. 8 These changes in the 

 gaseous content of the water surrounding the plant lessen 

 the capacity of the water to hold the dissolved calcium 

 carbonate and this is precipitated about the bases of the 

 algae and upon their thalli. The precipitated material 

 has a laminated structure which probably results from 

 variations in the rate of precipitation and interruptions 

 thereof. The process is not one wherein the lime carbo- 

 nate enters into the tissues of the plant and hence the 

 precipitated material may develop little or no internal 



8 Davis, C. A. : Geol. Surv. Mich., vol. 8, pt. 3, p. 69, 1903. 



