C.—GEOLOGY 81 
changes, or accidents (forest fires, etc.), is most illuminating. Now 
palzontologists had noted the possibility long ago, Witham, Lindley and 
Hutton and the earlier writers on the internal structure of fossil plants 
had noted, and discussed the implications of, the fact that in Palaeozoic 
times certain specimens of one and the same species might have rings 
in the wood, while others might not. Unger in 1847 noted that Mesozoic 
woods (Lower Triassic) had poorly developed rings, and therefore 
concluded that the equable climate of Palzozoic times was becoming 
periodic; but later workers, Arnold, for example, have proved that in 
Callixylon from Upper Devonian rocks, rings were quite well developed, 
and consequently the climate of Paleozoic times was not equable. 
Botanical research shows that one and the same species may or may not 
have rings, depending on the conditions in which the specimens were 
growing. ‘This, in some cases, depended on whether the plants were 
growing in warm or cold places, but, in others, on whether the climate 
was equable or not, and quite irrespective of any particular climatic zone. 
The reaction was to environment, but not necessarily to seasonal changes. 
In other words, the rings are difficult to interpret. But some plants are 
specially sensitive to these changes, and plants in temporate regions— 
or at the higher elevation in tropical lands—nearly all have these rings. 
Conifers have the most distinct rings, in general, and even Araucaria, 
where they are not so marked, produces rings under varying conditions 
of nutrition. Now conifers have a long geological range, and so might 
possibly indicate seasonal rhythm in past times ; or even indicate, by breaks 
in the rhythm, some exceptional occurrences that might be rhythmical or 
not. Antevs °° after recalling all the difficulties and the need for caution, 
has stated, ‘We can say with certainty that the occurrence of very marked 
zones in Jurassic woods from Spitzbergen, and the lack of rings in Jurassic 
woods from British East Africa, indicates marked climatic zones and 
pronounced annual periodicity in Jurassic time.’ Douglas made a very 
careful study of the ‘ Big Trees’ in the Sierra Nevada and other areas in 
America, and concludes that any ‘index’ tree must be very carefully 
selected, and the results checked not merely in the immediate vicinity, 
but consistent records must occur further afield. He found that the 
best index tree is the Yellow Pine, and the next best the Scotch Pine. 
Sequoia gigantea, while more complacent to changes than the others, is 
longer lived, and the records from these trees may go back to 1,000 B.c. 
with consistent results over a considerable area. 
While geologists will not benefit much from these researches directly, 
yet meteorologists and archzologists will, and a cross check on de Geer’s 
results from Varve counts in Scandinavia, or similar results in America, 
may yet be effected. ‘The age of ancient ruins at Gobernador Canyon, 
Aztec, and Pueblo Bonito, Chaco Canyon, Aztec, New Mexico, have 
been ascertained by examination of the ring record of logs which still 
retained their bark, and which had been used in building these dwellings. 
This was done by reference to large trees in the area, and a count back 
until the ring record of the log and that of the ‘ index’ tree coincided. 
80 Amer. Jour. Sc., vol. ix, p. 300 (1925). 
