John Gimn — Changes of Climate. 125 



are not faults, in the ordinary sense of the word, but heaves due 

 to the contraction of an individual bed or of a series of beds. An 

 oblique riban in certain argillaceous beds, has already been discussed 

 in a paper read before the Geological Society of Dublin (Nov. 11, 

 1863). 



VII. — On Changes of Climate during Long Periods of Time, 

 AND THE Conjoint Action of Precessional Movements and 

 OF THE Elevation and Depression of Mountain Eanges in 

 producing them. 



By John Gunn, M.A., F.G.S. 



IN my former Paper (see Geological Magazine, 1884, Decade 

 III. Vol. T. pp. 73-78) I endeavoured to show that as the 

 elevation of mountain-ranges caused cold, so their subsidence in long 

 periods of time was the cause of a warm temperature. 



Astronomical agencies were expressly excepted from consideration, 

 because they are independent of, and are the same under all changes 

 •of the level of the land and the consequent changes of the Fauna 

 and Flora. 



My object in this paper is to point out what those astronomical 

 agencies are, such as the Precessional and Perihelionic Cycles, which 

 are constantly in operation. 



They are well described in the late Mr. Mitchel's popular work, 

 "The Orbs of Heaven," p. 116, as follows : — 



" The line of equinoxes divides the earth's elliptic orbit into two 

 unequal portions. The smaller part is passed over in the fall and 

 winter, causing the earth to be nearer the sun in this season than in 

 summer, and making a difference in the length of the two principal 

 seasons, summer and winter, of some seventeen and a half days. This 

 inequality, which is now in favour of summer, will eventually be 

 destroyed, and the time will come when the earth will be furthest 

 from the sun during the summer, and nearest in the winter. But at 

 the end of a great cycle of more than 20,000 years, all the changes 

 will have been gone through, and in this respect a complete com- 

 pensation and restoration v/ill haA'e been effected." 



Thus he describes the Precessional Cycle, and then proceeds to 

 give the following account of the Perihelionic. " The Precessional 

 epoch of subordinate restoration will find the perihelion of the 

 earth's orbit located in space far distant from the point primitively 

 occupied. Five of these grand revolutions of upwards of 20,000 

 years must roll round before the slow movement of the perihelion 

 shall bring it back to its starting-point. 110,000 will then restore 

 the axis of the earth's orbit and the equinoctial line nearly to their 

 relative positions to each other, and to the same region of absolute 

 space occupied at the beginning of this grand cycle." 



Next Mr. Mitchel describes the still more extended cycle of all 

 the Planetary orbits, when all their perihelia meet together in their 

 original position. " If now," he says, " we direct our attention to 

 other planets, we find their perihelion-points all slowly advancing 



