NA TURA L HIS TOR Y OF MARL 495 



in lakes to which much silt is brought by inflowing streams, or 

 which have exposed shores where the waves are constantly cut- 

 ting and stirring up rock debris, the more slowly accumulating 

 marls will be either so impure as to be worthless, or so obscured 

 as to escape notice altogether, even where Chara is abundant. 

 It may also be pointed out that shallow water, strong light, and 

 a bottom of either clay, sand, or muck, present conditions favor- 

 able for the growth of the higher vascular plants, and that these 

 cause such rapid accumulation of vegetable debris that the cal- 

 careous matter may be hidden by it, even when Chara is a well- 

 marked feature of the life of a given lake. 



Another plant form, like Chara an alga, but of a much lower 

 type, which is concerned in the formation of marl is one of the 

 filamentous blue-green algae, determined by Dr. Julia W. Snow, 

 of the University of Michigan, to be a species of Zonotrichia, or 

 some closely related genus. 



The work of this species is entirely different in its appear- 

 ance from that of Chara, and at first glance would not be attrib- 

 uted to plants at all. It seems to have been nearly overlooked 

 in this country at least by botanists and geologists alike, as but a 

 single incidental reference to it has been found in American lit- 

 erature. 1 Curiously enough, however, material very similar, if 

 not identical, to that under consideration has been described 

 from Michigan in an English periodical devoted to Algae. 2 In 

 this the alga is identified as Schizothrix fasciartata Goment. As 

 comparison of material is not possible at the present time, the 

 plant under consideration is here tentatively called Zonotrichia. 

 The plant grows in relatively long filaments, formed by cells 

 growing end to end, and as they grow, the filaments become 

 incased in calcareous sheaths. The feature of the plant which 

 makes it important in this discussion, however, is its habit of 

 growing in masses or colonies. The colony seems to start at 

 some point of attachment, or on some object like a shell, and to 

 grow outward radially in all directions, each filament independent 



1 McMillan : Minn. Plant Life, 1899, p. 41. 



2 G. Murray : Phycological Memoirs No. XIII, 1895, p. 1, PL XIX. 



