This brings us to the end of the main hypotheses as to climatic changes, aside from the solar cyclonic hypothesis which will be discussed in the next chapter. It appears that variations in the position of the earth at perihelion have a real though slight influence in causing cycles with a length of about 21,000 years. Changes in the carbon dioxide of the air probably have a more important but extremely slow influence upon geologic oscillations. Variations in the size, shape, and height of the continents are constantly causing all manner of climatic complications, but do not cause rapid fluctuations and pulsations. The eruption of volcanic dust appears occasionally to lower the temperature, but its potency to explain the complex climatic changes recorded in the rocks has probably been exaggerated. Finally, although minor changes in the amount of heat given out by the sun occur constantly and have been demonstrated to have a climatic effect, there is no evidence that such changes are the main cause of the climatic phenomena which we are trying to explain. Nevertheless, in connection with other solar changes they may be of high importance. FOOTNOTES: [Footnote 10: James Croll: Climate and Time, 1876.] [Footnote 11: T. C. Chamberlin: An attempt to frame a working hypothesis of the cause of glacial periods on an atmospheric basis; Jour. Geol., Vol. VII, 1899, pp. 545-584, 667-685, 757-787. T. C. Chamberlin and R. D. Salisbury: Geology, Vol. II, 1906, pp. 93-106, 655-677, and Vol. III, pp. 432-446. S. Arrhenius (Kosmische Physik, Vol. II, 1903, p. 503) carried out some investigations on carbon dioxide which have had a pronounced effect on later conclusions. F. Frech adopted Arrhenius' idea and developed it in a paper entitled Ueber die Klima-Aenderungen der Geologischen Vergangenheit. Compte Rendu, Tenth (Mexico) Congr. Geol. Intern., 1907 (=1908), pp. 299-325.