C.—GEOLOGY 71 
interested in these floras, and must be congratulated for the way its workers 
have given their energies to these problems. Prof. Seward, Dr. Harris, 
Dr. H. Hamshaw Thomas, Prof. Walton and others have contributed 
noteworthy memoirs, not merely describing the plants, but discus- 
sing the geological and botanical implications of the floras they have 
studied. This has necessitated devising new methods of examination, 
and Dr. Thomas’ work in that regard is most valuable. Indeed it has 
led to recent investigations of Paleozoic fructifications, at the hands 
of Dr. Halle, that are most illuminating, and that have already been 
mentioned. 
In general terms the Mesozoic floras contain a few survivals from 
Paleozoic times, but the special development of new forms of ferns, 
_ possibly some pteridosperms, cycads, conifers and rare types that may 
be the percursors, or even the ancestors, of the flowering plants, are the 
main features of the floras. On the whole, the interest of the Mesozoic 
floras is botanical rather than geological. So far as I know, no great 
amount of zoning has ever been accomplished by using these plants. 
Yet Pia has employed algz to determine zones in the Alpian Trias. 
If the Palzozoic flora has drawn attention to world climates in the past, 
that of Mesozoic times has accentuated the position. In point of fact it 
was the discovery of Mesozoic plants in Arctic regions that drew attention 
to the problem, if not in the first instance, at any rate at an early date. 
A brief consideration is therefore not only warranted, but imperative, in 
our study this morning. 
In some areas where the Gondwanaland flora has been developed, and 
particularly in India and Australia, there seems to be a gradual change from 
the late Paleozoic Glossopteris-Gangamopteris flora into that of Mesozoic 
times and terminating in the Thinnfeldia flora. In Europe the plant 
series in similar rocks is scanty, and in America enormous numbers of 
coniferous trees, that are represented in the petrified forests of Arizona 
and Utah, are derived from one horizon—the Chinle formation of Middle 
Triassic age. In these special areas in India and Australia there are 
apparent links with the upper Palzozoic vegetation, but our knowledge 
seems like ignorance, in contrast with what has been discovered in the 
Coal Measure flora. Much more work is necessary. 
It is the Jurassic-Lower Cretaceous flora, however, that has attracted 
most attention in botanical circles; for it is a phase that was suddenly 
replaced by one closely similar to that of the present time, and yet one 
that was itself quite distinctive. Perhaps the most striking character 
is the number and variety of plants of the cycad class. But it was a 
complete plant phase, with all the main groups represented, and even the 
flowering plants are heralded in the Caytoniales of the Yorkshire deposits. 
The Rheetic flora has been brought to our notice recently by the wonder- 
ful suite of plants of this age from Greenland. The flora in general shows 
a considerable development of ferns of the Osmunda type, numerous 
cycads and other gymnosperms, among which the Ginkgoales (Ginkgoites, 
Baiera, etc.) are specially prominent, and the genus Sagenopteris, which 
Dr. Thomas has shown probably bore fruits that are a first approximation 
to those of the flowering plants. In the later Jurassic rocks distinctions 
