330 PAST CLIMATES AND CLIMAXES. 



to year and decade to decade, but in much larger ones, which take hundreds of 

 years or even thousands. These in turn seem to merge into and be imposed 

 upon the greater waves which form Glacial stages. Glacial epochs and Glacial 

 periods." (L c, 529) 



Climatic changes, then, are assumed to be always related in cycles. No 

 change stands out as a separate event; it is correlated with a similar event 

 which has preceded it, and one that has followed or will follow it, from which 

 it is separated by a dissimilar interval. Climate may thus be likened to a 

 flowing stream which rises and falls in response to certain causes. It is not a 

 series of detached events, but an organic whole in which each part bears some 

 relation to the other parts. Considering climate as a continuous process, it 

 follows that we must recognize changes or variations of climate only as phases 

 or points of a particular climatic cycle, which lose their meaning and value 

 uiJess they are considered in connection with the cycle itself. It is in this 

 sense that changes and variations are spoken of in the following pages, where 

 the cycle is regarded as the climatic unit. 



Kinds of cycles. — It is obvious that all cycles agree in being characterized 

 by phases of increase and decrease, and that all but the smallest are made up 

 of minor cycles. They vary markedly in intensity, duration and the mmiber 

 of included cycles, as well as in the climatic factors involved. They differ 

 also with respect to causes, especially the primary one, and with respect to the 

 area concerned. In this last respect, cycles may be distiuguished as general 

 or regional, and perhaps also as primary or secondary. At present the most 

 convenient distinction is based upon dm-ation, which necessarily includes 

 intensity in a large degree. Upon this basis there may be distinguished cycles 

 of 1, 2.5, 11, 21, 35, 50, 100, 400, and 1,000 years, approximately. The 

 annual cycle may be left out of consideration, except in its relations to other 

 cycles. In addition, there are larger cycles of unknown duration, such as 

 Gilbert has shown for Lake Bonneville, such as are represented by the glacial 

 and interglacial cycles of a glacial period, and the great cycles of cooled or 

 cold and warm periods of the eras. 



It is also convenient to distinguish cycles with reference to their cause, as 

 deformational, solar, or volcanic climatic cycles. Such a distinction must be 

 used with care, however, since many cycles must be due to the action of two 

 or three causes. Its chief use lies in emphasizing the primary cause, as in the 

 great cycles of eras, or in designating the cause which produces a cycle within 

 a cycle, as a volcanic cycle within a major sun-spot cycle. It is likewise 

 necessary to recognize major and minor cycles for purposes of comparison. 

 Major cycles, moreover, are characterized by minor cycles of varying degree. 



Arctowski's cycle of 2.5 years. — ^While this is the shortest and least important 

 of the climatic cycles, the work of Arctowski (Huntington, 1914 : 233, 243) 

 seems to indicate its existence as fairly certain. According to Huntington, he 

 has foimd areas of abnormal pressure, temperature, rainfall, and the like, which 

 persist for several years and move irregularly ,backward and forward. Regions 

 where the mean temperature for a given period is above or below the normal 

 are not distributed irregularly, but with much system. The excess of tempera- 

 ture is greatest at one point, from which it decreases gradiially to the area of 

 normal temperature, which gives way to a deficiency that centers aroimd a 

 definite spot. The regularity is so great that lines of excess or deficiency can 



