42 
Geology oe Sydney. 
stalked along the margins of an inlet before the human 
race was born pressedfootprintsin the soft and shifting 
sand which the rising and sinking of the continent 
could not wipe out.” 
Sometimes not merely impressions but the whole 
skeletons of animals long since extinct are preserved 
as fossils. Almost complete skeletons of the extinct 
giant marsupial Dijnotodon were found in great abund- 
ance at Lake Mulligan 1 . Fossils are preserved in 
somewhat different methods. (1) In rare instances the 
original substance may be preserved. In the case of 
coal the carbon present is the carbon of which the 
plants were formed. (2) The forms of shells are some- 
times preservedas casts and moulds. (3) The substance 
may be entirely lost, and the form preserved completely 
in another material. The so-called "petrified shells” 
come under this heading. Also "wood turned into 
stone” — si 1 icified wood of geologists. In the case of 
silicified wood, the whole substance has been changed 
from wood into stone, but every structure and fibre, as 
well as the rings of growth, are perfectly preserved. 
It is, says Jukes, "as if a house were gradually 
rebuilt brick by brick, or stone by stone, a brick or a 
stone of a different kind being substituted for each of 
the former ones, the shape and size of the house, the 
forms and arrangements of rooms, passages, and closets, 
and oven the number and shape of the bricks and stones 
remaining unaltered.” Sir Charles Lyell has also dealt 
with this most interesting question, and thus explains 
1 Laki) in Centra' Australia, but in South Australian territory. 
