SIB CHARLES LYELL. 235 



must pass in very high latitudes, not to speak of the greater prevalence 

 of cloud in reo - ions round the Pole. A truer cause of climatic change 

 Is to he sought in the effect of precession of the equinoxes, the revolu- 

 tion of the apsides, and, above all, the eccentricity of the earth's orbit. 

 The great cycle of change due to precession would cause the different 

 seasons of the Northern and Southern Hemispheres to coincide in turn, 

 within 25,868 years, with all the points through which the earth passes 

 in its. orbit round the sun. Combining with this movement, that of 

 the revolution of the apsides or " motion of the aphelion," as Herschel 

 named it, reduces this term of years to about twenty-one thousand. 

 Sir C. Lyell's explanation, aided by a new diagram, renders sufficiently 

 clear the effects which would be produced upon climate by the succes- 

 sive phases of precession, especially when combined with increased ec- 

 centricity or distance from the sun. The difference between winter in 

 aphelion and perihelion the range of eccentricity extending, as he has 

 shown, to 14,000,000 miles at some periods, instead of 3,000,000, as now 

 is set down by Mr. Croll as not less than one-fifth of the entire heat 

 received from the sun. Some slight change in this direction since the 

 year 1248 a. d. has been thought capable of actual proof by M. d'Adhe- 

 mar, and of being verified by the observations of M. Venetz upon the 

 decrease of Swiss glaciers prior to the tenth century, and their subse- 

 quent increase. An admirable table compiled by Mr. Stone shows the 

 variations in eccentricity for a million years before 1800 a. d., with the 

 number of days which would be added to winter by its occurrence in 

 aphelion, which has been followed up for a million years more by Mr. 

 Croll and Mr. Carrick Moore. From these figures there might appear 

 to be a possibility of approximating to a date for the Glacial epoch ; 

 and Sir C. Lyell holds it " far from startling " that 200,000 years back 

 might be fixed upon as about the period of greatest cold, when the ex- 

 cess of winter days amounted to 27.7. He had in his tenth edition 

 speculated upon 800,000 or 1,000,000 years as nearer the Glacial epoch, 

 but he feels compelled to narrow the time within the limit at which the 

 principal geographical features of the continents and oceanic basins 

 were approximately assuming their present form. "Were the astronom- 

 ical theory, however, to be relied upon as the basis for the solution of 

 the problem, we ought to meet in the course of palreontological research 

 with a series of Glacial periods perpetually recurring in the Northern 

 Temperate Zone ; supposing a large .eccentricity by itself sufficient 

 apart from the cooperation of terrestrial causes, to intensify the cold 

 of high latitudes. But no such evidence of violent revolutions is to be 

 found in the flora and fauna of earlier periods. The continuity of forms, 

 particularly in the class of reptiles, from the Carboniferous to the Cre- 

 taceous period, is an obvious fact opposed to the intercalation of intense 

 glacial epochs. Another fact is, that many great cycles of eccentricity 

 must have been gone through in the long centuries of the Carbonifer* 

 ous period, in which no break in the order of life is manifested, 



