10 Rev. LE. Hill—Eccentricity and Glacial Epochs. 
Il.—Eccrntriciry anp Guactat Epocus. 
By the Rey. H. Hitt, M.A., F.G.S. ; 
Fellow and Tutor of St. John’s College, Cambridge. 
R. CROLL’S “Climate and Time” is perhaps the most important 
geological work that has appeared in recent years, certainly 
the most valuable and original that refers to Glacial Geology. It 
teems with novel and instructive ideas. The part which is best 
known and has chiefly contributed to its celebrity is the theory, 
propounded and maintained in the first four chapters, of the causes 
of the Glacial Epoch. ‘Though the theory met with much opposi- 
tion at first, it seems now to be gradually gaining belief. Not only 
are Dr. Croll’s views accepted by geologists, but his arguments are 
quoted in text-books. Now, when Science is used as a means of 
education, it is of high importance that the reasoning placed before 
the student should be sound. I propose in this article to examine 
Dr. Croll’s description of the manner in which, according to his views, 
Glacial Periods have been brought about, to discuss the soundness 
of his reasoning, and to investigate the actions of the causes which 
he suggests. Some of the remarks here made are probably origin- 
ally derived from an able article by Mr. 8. V. Wood, jun., in the 
Grou. Mac. for 1876. The suggestion at the end of the paper is 
due to Mr. W. M. Hicks, M.A., Fellow of St. John’s College. 
Dr. Croll’s theory, as developed in Chapter IV. of ‘ Climate and 
Time,” is, that a Glacial Period was a period during which the 
eccentricity of the earth’s orbit was near its highest value, and the 
position of the Earth’s axis such that the Pole of the hemisphere 
under consideration was leaning away from the sun, and in darkness 
at the time of the year when the earth and sun were at their greatest 
distance apart. In astronomical language, it happened when, during 
a period of maximum eccentricity, winter and summer coincided with 
aphelion and perihelion respectively. In support of this theory he 
reasons as follows. When the eccentricity is larger, the winter will 
be longer than it is at present, and, owing to the sun’s greater dis- 
tance, also colder. Thus, at the commencement of the short summer 
the ground will be covered with snow and ice to an extent beyond 
what we at present experience. The summer’s heat will be unable 
wholly to melt this; the quantity at the beginning of the next sum- 
mer will be even greater than before, and the snow or ice will 
continue to accumulate year by year, so that with sufficient time 
any quantity whatever may be reached. The summer heat will be 
unable to melt the winter’s snow, he says,—First, by reason of the 
lowered temperature, lowered “by means of direct radiation” (that 
is, so far as I can understand, the masses of snow chill the air, 
and so render it incapable of melting them). Second, by reason 
of the reflective power of snow and ice. Third, owing to the thick 
fogs which are generated by the chilly masses, and shield them 
from the sun. Fourthly, by the deflection of ocean currents which 
would result from intenser trade-winds brought about by the 
intenser cold which would invade the pole of the hemisphere 
considered. 
