112 THE GLACIER EPOCH OF AUSTRALASIA, 
astronomical periods of extreme eccentricity with winter 
in aphelion, alternately every 10,500 years in each hemi- 
sphere. TZhere ts not, however, the slightest evidence of glactal 
epochs, such as that of the pletostecene period in Europe and North 
America, occurring within the tertiary period, corresponding 
to the recurrence of cycles of extreme eccentricity. Indeed, 
the double or repeated recurrence of a glacial epoch within 
the pleistocene period in the Northern Hemisphere, and pro- 
bably the glacier epoch towards the close of the tertiary 
period in Australasia, appear to be peculiarly exceptional oc- 
currences within the whole range of the Cainozoic period. 
Both Dr. Croll and Sir Robert Ball clearly perceived the 
difficulty presented by the lack of evidence of earlier glacia- 
tion in our rocks corresponding to former recurrences of 
cycles of maximum eccentricity. The latter dismisses this 
serious difficulty far too curtly, by allusions to the alleged 
imperfection of the records of the rocks, and to the perishable 
nature of boulder clays, rendering them peculiarly hable to be 
washed away; and, as regards moraines, erratics, etc., he 
adds, ‘‘ The advent of one ice sheet ploughs away the traces of 
preceding ice sheets.” But surely some definite traces ought 
to be found intercalated among the many well-preserved 
beds of the equally perishable sediments of the tertiary 
formations within the region covered or affected by the last 
great spread of ice during the pleistocene age. References 
to the rate and amount of denudation by atmospheric and 
other causes, based upon the amount of sediments held in 
suspension, and solutions derived from the waste of the land 
in rivers flowing into the ocean, may be fairly correct, but 
surely this waste is not composed entirely of the latest formed. 
deposits. The destruction ever going onin our rocks does 
not operate so intensely upon the latest layers formed as 
upon particular areas whose slopes and troughs favour the 
rapidly erosive action of the great destroyer, water im motion > 
and this action operates in vertical cuts and gashes through 
the envelopes of whatever strata may be underneath, rather 
than in sweeping away all trace of the most recently formed 
layers, many of which must. occur in such situations where 
they were covered and permanently protected by the newer 
sediments in course of formation, and, perhaps, largely 
derived from the immediate waste of the very oldest rocks. 
Why, therefore, should we not expect fairly complete vertical 
fragments of ancient boulder clays, moraine stuff, erratic 
drifts and in the tertiary formations at least, as commonly 
as we do of contemporaneous clays, deposited gravels, lignites 
and sediments, equally perishable stuff, otherwise derived ? 
This is not the only difficulty which bars the way to the 
acceptance of the astronomical theory taken by itself in 
adequately accounting for the glacial epoch of Northern 
