36 
Geology of Sydney. 
in another allowing it to subside. Now it is violent 
and convulsive, bursting asunder the solid rocks, and 
shaking the foundations of the hills ; again, it is gentle 
and harmless, upheaving vast continents with a scarcely 
perceptible undulation, not unlike the long, silent swell 
of the ocean. So it has been from the beginning, and 
so it is found to be even now, in this last age of the 
geological calendar. For, even within historic times, 
vast tracts of land have been permanently upraised 
by the convulsive shock of the earthquake ; and many 
parts of the earth’s crust have been subject to a slow 
wave-like movement, rising here and subsiding there, 
at the rate of perhaps a few feet in a century Some- 
times, too, the fiery liquid itself has burst its barriers, 
and poured its destructive streams of molten rock far 
into the peaceful, smiling valleys. 
