74 PRINCIPLES OF STRATIGRAPHY 



may have resulted from a discharge entering the bank of sand since 

 digging has exposed it, but it is difficult to understand why the dis- 

 charge should run parallel to the face of the cliff. 



It is obvious that fulgurites found in the older formations may 

 have such relationships that they must be regarded as contemporane- 

 ous in age with the formation, in which case they would form valu- 

 able evidence for the continental origin of such formations. 



CLIMATE. CLIMATIC ZONES, PRESENT AND PAST. 



Climate is the average or sum total of weather conditions normal 

 to a given region, while weather is only a single occurrence or event 

 in that series of conditions, and of short duration. Climate may be 

 classified as solar and pJiysical, the latter comprising continental, 

 marine, and mountain climates. 



Solar Climates. Climatic Belts or Zones. 



If the earth had a homogeneous land surface and no atmosphere, 

 solar climate alone would prevail, the distribution of heat being 

 solely according to latitude. Five zones of solar climate are recog- 

 nized : the torrid or tropical, limited by the tropics of Cancer and 

 Capricorn and divitled by the equator, the two temperate zones, lim- 

 ited poleward by the arctic and antarctic circles, and the two frigid 

 or polar zones centering at the poles. The torrid or tropical zone 

 has the least annual variation in insolation, and may be called the 

 zone of perpetual summer, while the polar zones have the greatest 

 variation between summer and winter, and may be called the winter 

 zones, summer being so short and cool that it may be neglected. The 

 intermediate zones have about an equal distribution of seasons, 

 summer in the northern corresponding to winter in the southern 

 hemisphere. A somewhat different delimitation of the zones has 

 been proposed by Supan, who suggests that the hot belt be limited by 

 the two mean animal isotherms of -f- 68° F. (+ 20° C.) (the ap- 

 proximate polar limit of the trade-wind belts, and the polar limit of 

 palms), and the polar limits of the temperate zones, by the two 

 isotherms of -|- 50° F. (-|- 10° C.) for the warmest month (the ap- 

 proximate northern limit of the cereals and forest trees). This 

 limit in the north is very near the northern boundary of the conti- 

 nent of North America and Eurasia, while in the south it passes 

 through the southern extremity of South America. Africa lies al- 

 most wholly within the hot belt, and so does the greater part of 

 South and all of Central America. The smaller size of the north 



