496 _Harrison—On the Geology of Hobart Town. 
but few evidences of changes going on elsewhere, or even in their 
immediate neighbourhood. f 
In speaking of the section passed over between Hobart Town and 
New Norfolk, mention has been made of certain recent aqueous 
deposits. Among these the most remarkable feature is the enormous 
amount of pebbles accumulated in many places. Such pebbles are 
of every size, from that of coarse grains of sand to boulders mea- 
suring many feet in circumference. They are composed of a variety 
of materials—quartz, granite, sandstones and limestones, basalt, 
diorite, and, in a few instances, what I judged to be fragments of 
Silurian slates. The whole of these are waterworn to a great extent. 
In some places an accumulation of pebbles only occupies the whole 
of an exposed section; but in many cases the pebbly deposit rests 
upon or is interstratified with loam, clay, or sand. 
One or two local ‘geologists suggested whether the pebble-bed 
might not be due to the former existence of glaciers. ‘The more 
likely cause, however, would seem to be one involving tidal action. 
The Derwent, as may. be seen on the map, is of very different width 
at different parts of its course; in some places it contracts into a 
narrow channel, and in others expands into a wide lake-like basin. 
As may be supposed, this conformation, by the expansions acting as 
reservoirs receiving and giving out the tidal wave, is productive of 
currents, running with great rapidity, quite sufficient to hurl onwards 
masses of stone as large as those spoken of. It is probable, too, 
that as the land gradually emerged from the sea, these irregularities 
in width may have been still more disproportionate than they are 
at present; or, on the other hand, such disproportion might, for a 
time, have disappeared altogether, as various heights above the sea 
were attained or exceeded. Hence we seem to have an ample ex- 
planation of the clay and sandy beds interstratified with what may 
be called the boulder-deposit. 
Scattered at intervals over much of the district near Hobart Town, 
are numerous beds of shells differing but little, if anything, from 
those still found in the adjacent seas. Some of these beds are met 
with at the height of many feet above the highest tides. Similar 
beds are seen in many places round the shores of Port Phillip Bay, 
and are supposed to indicate that a progressive elevation of the land 
has taken place at no very distant period, even if such elevation is 
not still going on. 
Igneous rocks, as may be supposed from what has been already 
said, have played no unimportant part in developing the beauties of 
Tasmanian scenery. These consist principally of basalt and green- 
stone, forming mountain-ranges, capping the tops of hills, or pro- 
truding, as dykes, from the clefts of sand or limestone. Owing to 
the unequal wearing of the two rocks, the igneous and the sand- 
stone, as much as to the tremendous disruptions, in some measure 
connected with outbursts of the former, Tasmania appears as a 
thickly-wooded Caledonia; and Hobart Town reminds the Scotch- 
man not a little of his much but not too greatly lauded Edinburgh. 
True, there is no Castle Rock ; but Knocklofty, 1,700 feet in height, 
