X1V The Naturalist in Nicaragua 
meteorologist will each find in these pages additions to his 
store of knowledge and abundant material for study. 
With all this, the work is not a mere catalogue of dry 
facts: it is eminently a readable book, bringing vividly 
before us the various subjects with which it is concerned. 
Minutely accurate in his description of facts and bold in his 
reasoning upon them, Belt covered so much ground that 
some of his theories have not held their own; but others 
have stood the test of time and been absorbed into the 
world’s stock of knowledge, while all bear witness to the 
singular grasp of his mind and have stimulated thought and 
observation—which is a great virtue in theories, be they 
true or false. 
It has been already stated that Belt devoted the scanty 
leisure of his last years to the study of the glacial period, 
entering with zest into the consideration of its cause, the 
method of deposition of its beds, and the time-relationship 
of man to it—complex questions on which his imagination 
had full scope, and which, had his life been prolonged, his 
patient accumulation of evidence might have ultimately 
led him to suggest answers that would have been generally 
accepted by scientific men. But the cause of the remark- 
able change of climate during those late Tertiary and post- 
Tertiary times known as the glacial period is still without 
a completely satisfactory explanation. In Belt’s day 
geologists were inclined to get over the difficulty of account- 
ing for the phenomena by any feasible terrestrial change 
by explaining them as the result of cosmical causes, and 
Croll’s theory of the increase of the eccentricity of the 
earth’s orbit was widely received among them. Belt, on 
the other hand, held that the cold was due to an increase 
in the obliquity of the ecliptic. But these astronomical 
explanations have not met with much acceptance by 
physicists; and so chemists have been turned to by some 
geologists for support of the hypothesis of the variation in 
the amount of carbon dioxide in the air, or of other altera- 
tions in the atmosphere, while others have gone back 
to the idea of geographical changes. That considerable 
oscillations of the relative levels of land and sea took 
place during the Ice Age has been now clearly estab- 
lished, and the general result of the investigations favours 
Belt’s opinion that the land during part of that period » 
