318 PAST CLIMATES AND CLIMAXES. 



by subordinate cycles, such as the glacial-interglacial cycles of the Pleistocene. 

 Cycles of still less intensity doubtless have their effect in the migration, 

 competition, ecesis, reaction, and dominance typical of serai development, but 

 the correlation of such causes and effects must await the general application 

 of quantitative methods. 



Evidences from stases. — ^The intercalation of coal stases in the series of 

 glacial beds in the Permian period of Australia appears to confirm what is 

 suggested by other facts. This is that the alternation of coal stases of the most 

 variable thickness with strates of sandstone, shale, etc., in the Coal Measures 

 of the Paleozoic and Cretaceous is partly due at least to climatic changes. This 

 is especially true for the thin seams of coal. It seems increasingly evident that 

 cycles of glacial and interglacial conditions are due to solar causes. If this 

 be granted for the nine or ten glacial horizons of the Australian Permian, then 

 the interglacial coal-beds must be explained also as a consequence of climatic 

 cycles rather than of minor deformations or oscillations. On this basis, the 

 numerous thin coal-seams of the Pennsylvanian and the Laramie, for example, 

 are readily explained without the need of invoking continuous crustal oscilla- 

 tion with a minor period of submergence for each. While submergence must 

 have occurred from time to time, it seems more probable to assume also the 

 existence during the Pennsylvanian, for example, of climatic pulsations similar 

 to those of the Permian, but of less intensity and concerned with moisture 

 rather than with temperature. From analogy with peat-forming swamps of 

 to-day, the Paleozoic swamps would have developed the luxuriant climax 

 forest during the relatively drier periods, and these would have been over- 

 whelmed and buried during the relatively wetter ones, to reappear with a 

 second drier period. At present such an assmnption is purely hypothetical, 

 but the rapid increase in oiu: knowledge of major climatic pulsations in remote 

 times, and of minor ones in recent times, makes it extremely probable that pul- 

 sations of all degrees have occurred at all times, as Htmtington maintains. If 

 this become generally accepted, it necessarily includes the repeated develop- 

 ment and destruction of coal-forming vegetation during the various coal 

 periods. As a consequence, the alternation of a thin seam or bed of coal with 

 a strate would come to mean the existence of a corresponding climatic cycle. 



Evidence from amiual rings. — ^The dependence of plant growth upon weather 

 or climate is too obvious to require comment. It has been recognized only 

 recently, however, that this correlation provides a measure of climatic vari- 

 ations from year to year, and hence of the minor climatic cycles of the present 

 and the immediate past. While the correlation affects all plants, it is evident 

 that those which record the growth of each year are much better adapted to 

 serve as indicators of climate. Such are trees, which have recently been em- 

 ployed by Douglass (1909, 1914) in his convincing demonstration of the relation 

 between the sun-spot cycle, rainfall, and the annual rings of growth. This 

 method of establishing and dating recent climatic cycles has been adopted by 

 Huntington (1914, 1914^), and has been applied to Sequoia, with the result 

 that oiu- knowledge of the alternation of svm-spot cycles has been carried back 

 over a period of several thousand years. The completeness of the record for 

 the immediate past seems presumptive evidence of the existence of such cycles 

 throughout geological time. Fortunately, we are not dependent upon infer- 

 ence, since secondary growth has occurred occasionally or regularly in trees 



