312 PAST CLIMATES AND CLIMAXES. 



existed then as to-day, the general gradation must have been of the slow, dis- 

 continuous valley type. As a consequence, the climax formation at any one 

 time must have been largely predominant, and in a bird's-eye view must have 

 appeared essentially intact. In bad lands, in gullies, and along the cutting- 

 banks of streams, it was constantly being removed, and as constantly redevel- 

 oped, though much more slowly. In areas of deposit, water was constantly 

 being replaced by land, and the latter covered with climax vegetation as a 

 consequence of succession. Hence, the cycle of erosion finds its successional 

 correspondence in the sere and the cosere. When it is relatively short and 

 uniform, it is marked by the sere. When it is longer and shows marked varia- 

 tions, it is characterized by the fragmented cosere. In a particular spot, the 

 fragments are temporal, i. e., they are successive beginnings of the sere, but 

 the most striking feature is the spatial fragmentation over the whole area, due 

 to the local occurrence of erosion and deposit. 



PAST CLIMATES. 



Interpretation. — ^The discovery of widespread evidence of Paleozoic glacia- 

 tion during the last decade or two has produced a reversal of opinion in regard 

 to the climate of geological times. The climate of the past, and especially of 

 the Paleozoic era, had heretofore been regarded as more or less uniformly warm 

 and moist down to the Pleistocene glacial period. It is now beUeved that 

 periods of cold and of drouth occurred repeatedly from the beginning of 

 Proterozoic times, if indeed they have not persisted in some degree throughout 

 all geological periods. Although geologists had applied Lyell's principles of 

 imiformity to geological processes for nearly a centmy, it has occurred to 

 them but recently that the same principles must necessarily apply to cUma- 

 tology. This new attitude has been expressed by Chamberlin and Salisbury 

 (1906:2:88) as follows: 



"There seem, therefore, to have been, in Paleozoic times, much the same 

 alternations of very uniform with very diversified climates that marked the 

 Mesozoic and Cenozoic eras; in other words, the alternations of climate seem 

 to have been of much the same order throughout the known eras." 



Chmate and climatic changes can be studied directly only when they occiu*. 

 The investigation of past climates is thus possible only through an under- 

 standing of the causes and effects of climates and climatic changes to-day. 

 Of these, the effects offer much more positive evidence than the causes, since 

 the major effects such as the evolution and migration of plants, the layers of 

 a stase or costase, the behavior of glaciers, the deposition of salt-beds, or the 

 oxidation of sedimentary deposits can be definitely studied at the present 

 time. Still, the investigation of the causes of climatic changes has made 

 remarkable advances during the past decade, and has now reached a point 

 where much greater knowledge of the causes and effects of climate is rapidly 

 definitizing our understanding of geological climates. As suggested under 

 the method of sequence, every new fact not only serves to indicate additional 

 facts in the sequence, but the facts already accepted and adopted into the 

 sequence also serve as a check in both directions. While the credit for the 

 present promising state of our knowledge belongs to a score or more of cli- 

 matologists and geologists (c/. Huntington, 1914, 1914''), the most suggestive 



