186 BOTANICAL GAZETTE [SEPTEMBER 
may be seen meeting the Boutcloua formation. All along this ridge 
the thicket formation has dropped out and the pines advance alone. 
The forest is not dense, and while many of the principal and secondary 
species are found on its floor, plants are also found which belong to 
the grass formation and which are able to survive in this location. 
In fact, the grass formation is found here in places dominating the 
floor of the pine formation—a true mixing of the formations (fig. 10). 
This mixing may be due to a certain extent to the entrance of the 
dominant species of the grass formation, but it is more likely to be 
the result of the gradual advance of the pines into the grass forma- 
tion. This is shown very clearly in places where the young pines 
are several meters in advance of the older trees (fig. 11). This inva- 
sion may be observed along the ridge leading eastward from Palmer 
Lake and on which is found the so-called “black forest.” Here 
the pines and the grasses mix and there are no shrubs present (jig- 1 0). 
The principal species, Arctostaphylos Uva-ursi, is also pushing out 
into the grass formation. A short distance west of Pring, Colo., 
this pine formation may be seen rapidly advancing along an old 
roadbed. 
An entirely different condition may be observed eleven miles 
east of Colorado Springs. Here the pine formation is also advancing, 
but it is accompanied or rather preceded by the thicket formation. 
The advance of these two formations is favored by the cutting back 
of the gullies, forming steep hillsides, which offer the most favorable 
conditions for the growth of these two invading formations. 
In many places the thicket, pine, and grass formations are found 
to meet and mix equally, the grasses forming the floor between the 
shrubs, and the pine scattered throughout. These three important 
formations are not only found meeting here on equal terms, but 4 
remnant of an older formation, or at least one which gives evidence 
of greater age, is also found. This is the foothill woodland. Jun 
perus monosperma is scattered here and there and isolated trees of 
this species are often found which seem to be very old. _ Still stronger 
evidence is found in the fact that here, many miles removed rom 
its fellows, is a very large and apparently very old Pinus edulls. 
Erosion has removed the soil from the base of the tree, exposing " 
roots, and it is certainly much older than any of the other trees 
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