40 
Geology of Sydney. 
peculiar to the dry plain, another to the swampy morass ; 
as one family belongs to a temperate, another to a 
tropical region, so, from the character of the imbedded 
plants, we are enabled to arrive at some knowledge of 
the conditions under which they flourished. In the 
same manner with animals, each tribe has its locality 
assigned it by peculiarities of food, climate, and the 
like ; each family has its own peculiar structure, for 
running, flying, swimming, plant-eating, or flesh-eating, 
as the case may be ; and by comparing fossil remains 
with existing races, we are enabled to determine many 
of the past conditions of the world with considerable 
certainty. 
It is a well-ascertained fact that each period in the 
world’s history was marked by the existence of an 
animal and a vegetable creation peculiar to itself, and 
so characteristic of certain formations are certain 
fossils that the geologist can very often determine the 
exact age of, and place in its geological sequence, a 
formation he has never seen, provided a collection of 
its fossils is placed before him. When a geologist finds 
a specimen of Glossopteris , for instance (fig. 38), in New 
South Wales, he can safely conclude that the rocks he 
is investigating are newer than Devonian, and older 
than Triassic, and that they belong to the series that 
contain the workable coal seams of the colony. 
Macrotxniopteris (fig. 4) characterizes beds lying 
above the productive coal measures. It tells of a for- 
mation that accumulated on the land, and not in the 
sea. Thinnfeldia (fig. 30) is peculiar to a formation 
