GEOLOGY 
which only the grains of desert sand ever take, and which at once serves 
to distinguish grains of sand so formed from similar grains worn by 
moving waters. The prolonged action of the wind acting upon large 
bodies of sand served eventually to pile the sand into great dunes, whose 
long axes in Cumberland ran north and south (as if their form were 
determined by the earth’s rotation) and whose steeper sides faced to the 
west. It is these old desert sand-dunes which now form the Penrith 
Sandstone, to be referred to in more detail presently. 
(c) With an irregular rainfall, small in total amount, there could be 
but little vegetation, and what little there was must have consisted of 
those species which had gradually adapted themselves to the arid condi- 
tions. Their nature is unknown, for no traces of them have been met 
with in the old sand-dunes ; but all analogy would seem to point to their 
being hard, thick-leaved and scrubby, with long roots adapted for exten- 
sion far down into what little damp soil they found. Probably many of 
them were armed with thorns, to enable them to hold their own against 
the few animals who were driven to use them as food. 
Animal life is always directly or indirectly dependent upon vegeta- 
tion, so that desert conditions have sometimes been defined as those which 
are unsuitable for carnivorous animals. Hence, whatever may have been 
the case in those parts of the world where humid conditions obtained, 
we may take it for granted that the only terrestrial forms of vertebrate 
life were those of animals whose structure enabled them to stand long 
droughts, and to travel easily from one part of the desert to the other 
where there happened to be suitable feeding-ground for the time being. 
So far as we can form an opinion from the only vestiges that are left, 
which are almost exclusively spoors of one kind or another, the animals 
in question were chiefly reptiles. If we may judge by the variety of 
these footprints, the animals must have been very diverse in form and size. 
Some appear to have been squat and thick, with perhaps the form of tor- 
toises. Others were more slender, and may well have been crocodilian in 
form. Others, again—and these probably represented some of the most 
advanced forms—were reptiles of kangaroo-like shape, with long and stiff 
tails, and with the hind limbs bigger than the fore, from which we may 
conclude that they occasionally progressed by the hind limbs alone. The 
zoological grade of some of these appears to have been intermediate be- 
tween some low vertebrate form with affinities not far removed from the 
Amphibia, and another belonging to the ancestral stock of the Mammalia, 
of a structural type not far removed from that of the Echidna and the 
duck mole of Australia of to-day. Many of the Reptiles of this period 
belong to the Anomodontia, whose structural characters place them in 
the systematic position referred to. These Reptiles are regarded as modi- 
fications derived from an Amphibian stock, which, in the course of long 
ages, and as a consequence of a gradual change in physical conditions, 
became adapted to a life exclusively upon land. Of mammals and birds 
there is no trace, and there is reason to believe that they had not yet 
come into existence. 
I 33 B 
