CLIMATIC CYCLES. 339 



from sun-spot cycles especially in the absence of maxima and minima which 

 pass gradually and somewhat regularly into each other. A glance at plate 57 

 shows, however, that there are periods of eruption and quiescence or of dust- 

 laden and dust-free upper levels, such as 1782-1789 and 1790-1797, 1808-1817, 

 and 1818-1829, etc., which are more or less cyclic in nature. The majority of 

 the eruptions appear to be episodic in character, however. In geological times 

 of great explosive activity, it is probable that the dust-blanket persisted for 

 much longer periods, and was followed by even longer periods of quiescence, 

 thus giving a major cycle of activity and rest. Such cycles seem to have 

 coincided with def ormational cycles, and to have merely increased or decreased 

 the cUmatic effect of the latter, as already noted. The smaller volcanic cycles 

 must have Ukewise coincided with various sun-spot cycles, and have had a 

 similar plus or minus effect. Finally, Arctowski has recently pointed out 

 that volcanic eruptions in the northern hemisphere probably have no effect 

 upon cUmates of the southern hemisphere, since their general atmospheric circu- 

 lations are independent. In this case volcanic cycles and episodes would further 

 differ from the great deformational cycles and especially from the sim-spot 

 cycles in not being universal in effect. 



Deformational cycles. — ^These have already been considered in some detail 

 (p. 302), and it must suffice here to distinguish the different cycles and their 

 correlations. On page 340, 9 major cycles have been distinguished, though 

 it is clear that they vary much in intensity and duration. Schuchert (1914 : 

 285) has indicated 22 periods of deformation, marked by similar differences 

 (fig. 26). While it is impossible to classify these with accuracy or finality, 

 there is good warrant for grouping them in three kinds of cycles. The grand 

 cycle of the eras is marked in general by the maximum deformations, though 

 the limits are not necessarily coincident with the deformation periods. As 

 has already been pointed out, deformation, with its clunatic, floral, and faunal 

 sequences, stretches over such a vast time that it conforms with the limits of 

 the accepted eras only in the case of the Proterozoic, when the life sequences 

 were all but lacking. The eral cycles are thought to be best shown by using 

 vegetation as a median sequence, as in the vegetation eras proposed on page 

 289. In short, it seems necessary to recognize that deformation may occur 

 in one era or period and its major effects may be felt in the next era or period. 

 Consequently, the 9 major cycles do not fit exactly into the 4 grand cycles. 

 Finally, there are a score, more or less, of minor deformational cycles, such 

 as occur within geological periods, and mark epochs or regions. It is fur- 

 ther possible to distinguish general epochal cycles from regional cycles, and 

 this will ultimately be done, though it is more or less hazardous at present. 



THE SERIES OF CLIMATIC CYCLES. 



Correlation of cycles. — Cycles of the same rank are sequent or serial, those of 

 different rank coincident or included. The grand or eral cycles fall into a 

 series consisting of the Eophytic, Paleophytic, Mesophytic, and Cenophytic 

 eras. The last of these, for example, comprises a series of major deformational 

 cycles, namely, the Cretaceous-Eocene, Oligocene-Miocene, and Pliocene- 

 Recent, which may m turn exhibit minor or regional deformational cycles. 

 The Pliocene-Recent cycle exhibits the period of Pleistocene glaciation as its 

 major feature, in which it is assumed that each glacial-interglacial sequence 

 corresponds to the grand sun-spot cycle. The correlation of the lesser sun- 



