ECOLOGICAL SIGNIFICANCE OF EAGLE CREEK FLORA 591 
climax forest of a mesophytic sort is estimated to be from one to 
two hundred years. This is on the basis of there being no soil at 
the outset, and of its development through the agencies of plants 
and weathering. It also assumes a soil favorable for the reception 
of plants. In the case of the soil furnished by the Eagle Creek 
rocks, there was a distinct time advantage due to the fact that 
they were not consolidated and therefore offered an immediate 
foothold for rooted plants. On the other hand the chemical 
composition of this sediment was probably quite unsuitable for 
the growth of most higher plants, certainly for the growth of 
mesophytes. The latter require a humus content which was 
entirely lacking in the original volcanic ‘materials. Further, due 
to its basic composition, this may be supposed to have been quite 
unfavorable for the development of such seedlings as germinated 
in it. Experimental evidence has shown that of seeds planted 
in pulverized Eagle Creek rock from several localities, only those 
of oaks (xerophytes) developed successfully. The experiments 
were not satisfactorily completed, and it is not known whether the 
oak seedlings would have continued to develop in this soil. Obser- 
vational evidence from regions recently covered by volcanic ash 
indicate that a number of years may elapse before the return of 
the higher plants. It is not unreasonable therefore to assume 
that the full one to two hundred years would have been required 
for the development of a climax mesophytic forest on the volcanic 
débris-strewn surface during Eagle Creek times. 
While it is not possible to correlate the various horizons which 
contain plant remains in widely separated parts of the area, due 
to their limited horizontal extent, it is possible to determine, on the 
basis of relative elevation, that there are at least ten distinct 
horizons represented. Each of these contains leaves of the climax 
forest which, as we have seen, would require from one to two 
centuries for its development. The total length of time involved 
in the growth of the ten plant horizons may thus be placed at from 
one to two thousand years. And when it is realized that there 
must be numerous other plant-bearing horizons which have not 
been uncovered, the length of the epoch as inferred from plant 
growth may be greatly extended. 
