REVIEW AND DISCUSSION. 135 
of physical conditions to which, in turn, correspond some of the most 
striking floristic differences observed on the Laboratory domain. ‘Thus 
the correlation of recent geological history and the movements of plants 
represented here becomes a matter subject to exact observation and record, 
as has been pointed out in previous sections. 
(3) The character of the rocks at this place does not appear, as a rule, 
to be intimately connected with differences of plant species, and this 
was to be expected from the general sameness of their chemical com- 
position. It is to be noted, however, that the rhyolite tuff, a conspic- 
uous formation of Tumamoc Hill, presenting a strong contrast to the 
dark-colored basalts, exhibits noticeable differences from the latter in 
its lichen flora, as has been pointed out by Professor Fink. The rapid 
formation of caliche described by Professor Tolman, in connection with 
what has been observed of its unfavorable character as a substratum 
for the growth of plants, may not improbably throw light on some facts 
of distribution not otherwise explained; but this has not yet been suf- 
ficiently investigated. 
(4) While emphasis has been laid upon the long-continued action of 
forces now in operation and upon the continuity of geological history, 
it is also necessary to take into account the fact that in the close vicinity 
of the Laboratory domain are areas the history of which extends farther 
into the past, and is more varied as regards climatic and other changes 
than is that of Tumamoc Hill. Whether this hill, however, was in a 
position to directly receive additions to its flora from the southward 
migration of the earlier Pleistocene, or received them later from neigh- 
boring areas is of comparatively little consequence. The evidence, chiefly 
botanical, that such migration took place is ample, and, by whatever 
way or means, ITumamoc Hill has received a share of the genera con- 
cerned in it. This part of the geological history of the region appears 
as yet to have been only partially made out, and until this has been 
accomplished no entirely definite account of its relations to the flora of 
the Laboratory domain can be looked for. 
It is seen from the foregoing that the critical study of a limited area 
from the geological standpoint, such as that presented by Professor 
Tolman, has proven to be of great value to the student of distribution, 
especially as regards general problems, and has also thrown light on 
various matters of local distribution. The present study furnishes abun- 
dant reason for enlarging the scope of the work and extending investi- 
gations of this kind more widely in the arid and semi-arid territory of 
the Southwest, where at this stage of botanical inquiry there is such 
pressing need of fuller knowledge of climatic conditions prevailing in 
later geological times. 
Through Prof. J. J. Thornber’s study of the local flora we are in pos- 
session of data bearing still more directly on the problem of the general 
