ORIGIN OF DEPOSITS. 523 
the highest inundations, may have been so low at another time as to 
have felt the rapid movement, and these changes of level were hence 
concerned in producing the alternations of material detailed on a for- 
mer page. ‘The large logs of Conifere in some of the layers, are 
evidence of a change like that here explained. If a region, which 
before was one of quiet depositions, were afterwards to come within 
the rapid part of the flood, in consequence of a subsidence, there would 
probably be other regions of quiet depositions produced at a distance 
elsewhere; and it is possible that evidence of this may yet be ascer- 
tained. 
There may be some reason for the very abundant vegetation during 
the coal era in Australia—a region which is now comparatively un- 
productive—in the probable fact, that a large amount of land was then 
becoming dry from the ocean, and thus favoured the production of 
clouds and rains; and also in the warmer state of the earth than at 
any later period. It is not impossible, moreover, that the composition 
of the atmosphere as, commonly supposed, favoured rapid growth. 
The subsidence in progress during the coal formation, ended finally 
in submerging the land beneath the sea, the condition of the region 
where the Sydney sandstone was forming, as is evident from the 
constitution and structure of the sandstone layers. 
Sand is to a great extent a seashore product, as we have remarked 
in another place. It is formed where the triturating waters have con- 
siderable motion, and where, therefore, the finer material derived from 
the constant wearing of the sands, is washed away. In still seas or 
quiet waters, the gentler action produces a fine mud. ‘The material of 
the Sydney sandstone bears evidence, therefore, of ocean origin, either 
on seashores like the sands of beaches, or in shallow waters off coasts. 
To this conclusion, the structure of the rock affords other support. 
The inchned layers of deposition, dipping so uniformly to the north- 
east in the neighbourhood of Port Jackson, point to a sea whose 
waves acted from that direction. At the same time, the thinness and 
delicacy of the structure, the changes in character, as if depositions 
once formed were afterwards partially removed, and were then again 
enlarged by new additions, show that the action of the waves was 
nearly constant and of varying force. In the changes presented by 
the rock, we may point out the very period in its progress when the 
action was for a while quiet, for we find there the structure becoming 
argillaceous, owing to the finer trituration. 
Were the inclined lamination less general and less regular in amount 
and direction, we might attribute it to local causes. But in fact it pre- 
