TRANSACTIONS. 85 
II. Theories of the Ice Age and Notes on the Glacial Geology of the 
immediate Neighbourhood. By Mr JAMES WATT. 
(Epitomised by the author.) 
The primary cause of great variations of climate is the 
position of the earth in relation to the Sun. 
Glacial conditions ensued in consequence of (a) a gradual 
increase of ellipticity of the earth’s orbit until it reached a period 
of maximum eccentricity, or extreme elliptical form; (4) the 
precession of the equinoxes, 
The combined effect of these two causes must, to a very large 
extent, influence the climate of the earth, because from precessional 
movement when the winter solstice in the northern hemisphere 
occurs when the earth is furthest from the sun in June, not as 
now in December, when it is nearest, and also, when from greater 
ellipticity of the orbit, the distance of the earth from the sun is, 
in the winter solstice, several millions of miles greater than now, 
then, in such circumstances, glacial conditions of excessive severity 
would prevail. 
Sir Charles Lyall, in his great work on the “ Principles of 
Geology,” showed conclusively that changes in the geography of 
the globe, combined with precession, would aecount for great 
changes of climate. But it was not till the appearance in 1864 of 
Dr James Croll’s remarkable paper, “On the physical cause of 
change of climate during Geological Epochs,” that it became clear 
that the primary cause of great variations of climate was astro- 
nomical. Dy Croll made calculations for the form of the earth’s 
orbit from the year 1800 for three millions of years back, and one 
million of years forward, calculated at intervals of 50,000 years, 
and his calculations have been verified by eminent mathematicians 
in Europe and America. According to these calculations, the 
period of the last great increase in the ellipticity of the earth’s 
orbit was reached 240,000 years back, and terminated about 
80,000 years ago, embracing a period of 160,000 years. That 
period we call the “Great Ice Age,” or “Glacial Epoch.” The 
cold was most intense about 200,000 or 210,000 years ago, and it 
is maintained by Croll and other eminent men that glacial condi- 
tions during the period of high eccentricity would not be continu- 
ous in northern and southern hemispheres, but that each hemisphere 
“toust have,” in Professor James Geikie’s own words, “experi- 
enced several’ great vicissitudes of climate. Glacial conditions 
