338 



PAST CLIMATES AND CLIMAXES. 



spond in the main. In 17 curves of tree-growth for different portions of the 

 United States, Huntington (1914 : 135) finds evidence of "long and important 

 cycles, having a periodicity of 100 or 200 years more or less, and affecting all 

 parts of the country." 



A major cycle of 300 years has been suggested by Clough on astronomical 

 groimds. A similar cycle of 300 to 400 years is shown by the maxima of 

 Huntington's Sequoia curve, as for example from 200 to 600 a. d., 600 to 1000, 

 1000 to 1350, 1350 to 1750 a. d. approximately, and perhaps by the minima 

 in the curve of changes of climate in Asia at 300, 650, and 1200 a. d. (1914^ : 530, 

 552). Moreover, Pettersson has pointed out a noted sun-spot maximum from 

 1370 to 1385, as Huntington shows (1914*: 552, 550). This was foUowed 400 

 years later by the greatest recorded maximmn, that from 1770 to 1790. 

 Furthermore, the Sequoia curve shows a great cycle of 1,000 years, from about 

 50 B. c. to 1000 A. D., and a somewhat similar cycle from 1000 to 1900 A. d., 

 broken by the maximiun of the fourteenth century (Huntington, 1914" : 530, 

 552) (fig. 34). 



B.C. A.D.IOO 200 300 400 500 600 700 800 600 1000 IIOD 1200 1300 MOO MOO 1600 noo 1600 



Fig. 34. — Major sun-spot cycles as shown by curve of growth of Segwyia trees in 

 California for 2,000 years. After Huntington. 



Finally, the glacial-interglacial epochs, or periods of advance and retreat of 

 the ice during the Pleistocene and the Permian, seem to represent the grand 

 sun-spot cycle. It appears impossible to explain such periods by either the 

 def ormational or the volcanic hypothesis. Deformation is the probable cause 

 of the glacial periods of the Pleistocene and Permian, but it is impossible to 

 connect it with repeated glacial epochs, while it seems equally difficult to 

 ascribe such a primary r61e to volcanic dust. Huntington (1914^:565) 

 assumes that glacial stages and epochs are due to solar changes, and has 

 advanced a plausible explanation of glacial periods, which is discussed later 

 (p. 369). Back of the record furnished by tree-growth, it is practically 

 impossible to determine the date of dm^ation of climatic cycles. If we use the 

 lowest estimates given by Chamberlin and Salisbury (1906 : 3 : 414, 421), 10,000 

 years represent the period since the disappearance of the ice, and another 10,000 

 years the retreat period of the Late Wisconsin. The latter is essentially an 

 interglacial epoch in its general nature, and this figure may be employed as 

 a very general approximation of the length of the later glacial and interglacial 

 epochs of the Pleistocene. The svun of the two, viz, 20,000 years, is regarded 

 as indicating in the roughest fashion the dvu-ation of the grand sim-spot cycle, 

 since, as indicated above, glacial periods, as distinct from epochs, are consid- 

 ered to be produced primarily by crustal deformations. 



Volcanic cycles. — ^These are phases rather than cycles, since they possess 

 little periodicity, except perhaps as secondary phenomena in connection with 

 the deformation cycles. Moreover, they are marked by times of great activity 

 followed by complete pause, usually of vastly longer duration. They differ 



