PERIODICITY OF RAINFALL 71 



tinental Kootanay and Cretacic deposits, especially the widespread 

 Dakota sandstone. As the west-winds descended on the eastern side 

 of the new Appalachians, they again produced semiarid conditions, 

 which favored the deposition of the continental Newark and Con- 

 necticut Valley sandstones and their extensions north and south, 

 formations which to-day are represented only by fragments of what 

 must have been formerly a very extensive piedmont deposit. South- 

 ward the condition seems to have been one of less aridity, possibly 

 because the mountains were lower, or south winds from the Atlantic 

 acted as modifiers, for here we find plant beds and coal deposits as- 

 sociated with the upper part of the formation. These same south- 

 westerlies were also responsible for the extensive continental de- 

 posits of the Trias of western Europe. A mountain chain, the Ar- 

 morican, extended northwestward from central France through 

 Brittany, southern England and Ireland, thus lying directly in the 

 path of the southwesterly winds. A second chain, the Variscian, ex- 

 tended eastward through southern Germany, around the Bohemian 

 Mass on the north, and thence to the present Carpathians. These 

 chains thus completely enclosed northern Europe on the south, and 

 to the north of them were formed the extensive New Red sandstone 

 deposits of England and the Bunter Sandstein with its desert fea- 

 tures in Germany. 



Latitude and Precipitation. Rain and snow decrease progres- 

 sively from the equator polewards, the precipitation being compara- 

 tively slight, owing to the smaller capacity of the air for water- 

 vapor at the prevailing low temperature, and to the absence of local 

 convectional storms. The western coast of Norway is an exception, 

 since the rainfall is here much heavier. The precipitation is mostly 

 in the form of snow^ though rain falls in summer sometimes prob- 

 ably at all points. The snow of the polar regions is fine and dry; 

 flakes are not formed at low temperatures, but the atmosphere is 

 full of fine, glittering ice needles, which gradually descend to the 

 earth even on clear days. 



Periodicity of Rainfall. In a number of places, as for example 

 India, observations have shown an apparent fluctuation in rainfall 

 and temperature for a period of eleven years, corresponding to the 

 sun-spot period or the periodicity of the earth's magnetic phe- 

 nomena. A thirty-five-year period of oscillation has also been dis- 

 covered and elaborated by Professor Bruckner (12) (Briickner 

 cycle). The cycle varies from 20 to 50 years, but 35 is the average. 

 The fluctuations of the rainfall between the dry and wet periods are 

 most marked in the interior of the continents. 



