1921| JOHNSON—VIRGIN SOIL e 307 
would be establishing themselves among those first invaders. 
Many of the possible invaders of the valley, found on the neighbor- 
ing hills, have a long growing season. Thére are some species that 
grow actively from February to September, while still others 
grow practically throughout the whole year.’ Because of this 
long growing season and the possibility of some humus washing 
down from the surrounding hills, it was assumed that by 1919 
the soil of the valley floor would be well hidden by a plant covering. 
—Looking east across Cascade Valley, gual sparse vegetation, which 
fe ae is caisk be still more evident if viewed from abov 
My surprise was great, therefore, when I found hardly more than 
a tenth of the gravelly bottom of the Cascade Valley hidden by 
plant foliage. The soil between these leafy plants, it is true, was 
not absolutely bare. There were a few very small patches of 
lichens and mosses. There was also a chroococcaceous alga, 
Gloeocapsa magma, which formed smooth encrustations often 
several square decimeters in extent on the pebbles and bowlders. 
This alga is present not only near the streams but across the whole 
floor of the valley. When dry Gloeocapsa has a rather dirty or 
chocolate brown color, but when wet it becomes a glistening velvety 
layer of a dark maroon color. It evidently thrives on these bare 
3 SHREVE, F., Publication no. 199, Carnegie Inst. Wash. pp. 51-52. 1914. 
