360 Annals of the Carnegie Museum. 



The Populus-Salix Formation. 



This formation, at the stage under discussion, shows but little change 

 from its structure in Stage D, other than in the growth of the cotton- 

 wood trees. The willow has almost entirely died out and a few of the 

 sand-plain species are beginning to crowd in around the bases of the 

 cottonwoods. 



Stage F. — Lagoon Eb. 



The middle one of the three marshy areas between Horse-shoe Pond 

 and Lagoon B presents an example of a stage in the succession some- 

 what further advanced than in Lagoon E, and there are quite a number 

 of changes to be noted in the different formations, as well as the ap- 

 pearance of one new formation. 



The structure of the vegetation of this marsh, as far as it is at pres- 

 sent represented, is as follows : 



(a) Scirpus-Typha Formation, 



((5) Cladhim-Calamagrostis Formation, 



(r) Rhus-Alnus Formation, 



(rt^) Populus-Salix Formation, 



The habitat of this series has been derived from a long narrow 

 lagoon, running parallel to the shore of the northwest end of Horse- 

 shoe Pond, from which this area as well as the two adjacent marshy 

 areas were derived, formerly having been lagoons (see Plate XLVIII). 

 In the middle one of these areas the lagoon has been filled to such an 

 extent that the lowest portion is inundated only at times of heavy rain- 

 fall. The vegetational zones have closed in on the lagoon, so that the 

 deeper central portion is now occupied by the Scirpus- Typha formation. 



The Scirpus-Typha Formation. 



This formation is here in a rather advanced stage, which is mainly 

 marked by the predominance of the Typha latifolia consocies. Scir- 

 pus americana is present, but is rather scarce and is not in a vigorous 

 condition. Of the three consocies of the formation the Typha lati- 

 folia consocies is evidently more partial to a humus soil, while Scirpus 

 americafia and Scirpus validus reach their best development in almost 

 pure sand. Drainage conditions perhaps enter into the problem to a 

 certain extent. 



Secondary species are here more abundant than in previous stages 

 and are as follows : 



