— 393 — 
conform well with the amelioration of the climate which seems to 
have followed the period of most extended ice action. On the whole 
it appears probable that the glaciation of Gondwana-land was due 
to several concurrent causes, chief of which were atmospheric 
reduction in carbonic acid and elevation of the continental surface. 
Concerning the date of the early Gondwana glaciation there is 
some difference of opinion though all are agreed that it falls within 
Palaeozoic or Permo-Carboniferous time. The principal evidence 
relating to this question is that furnished by paleeontology, the 
testimony of physical geology being itself modified and interpreted in 
terms of paleontological history. The evidence of marine invertebrates 
as to the age of the boulder beds is available at but few points, since in 
most regions the series containing the Gangamopteris flora are 
epicontinental. 
The marine beds associated with the glacial deposits in New 
South Wales have furnished fossils regarded by Frech (1) as Per- 
mian in age; and the’glacial series, lying generally unconformably 
on Lower Carboniferous or older rocks, is placed by them at the base 
of the Permian. The forms of Phyllotheca, Glossopteris and Noeggera- 
thiopsis found in the Greta series, a thin group of « Lower Coal 
Measures » apparently lying between boulder beds in this district, are 
believed to be the oldest representatives of the older Gondwana flora that 
have yet been discovered. 
The floras of Gondwana-land themselves present internal age 
evidence, in their generic relations and the identity of. the invading 
or transitional species. 
This is illustrated by the presence of Annularia in the Greta 
series just mentioned; Sphenophyllum in the Damuda of India and 
the Lower Karoo of Natal; Psygmophyllum and the reported Lepi- 
dodendron in the Transvaal; Annularia, Rachopteris, Pachypteris, 
Lepidodendron, and Waichia, reported by Bodenbender from Argentina ; 
and the Lepidophytes in Brazil. It is probable that the local species 
of some of these genera, such in particular as Annularia, are hardy 
offshoot descendants of the pre-glacial cosmopolitan representatives of 
the same genera, as no doubt is the case in Phyllotheca, which is present. 
in the Westphalian as well as the Permian of the North. 
(1) Lethaea Geognostica, Lethaea Palaeozoica, vol. II, Lief. 4, 1902, p. 590. 
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