Land round Melbourne. 229 
and with but little deviation from that parallel, thus forming 
the different granite ranges throughout the country. This 
mighty power, however, ultimately exhausted, a long period 
of comparative quiesence ensued, and the mud which was. 
hitherto in mechanical combination with the agitated waters, 
was deposited on the bed of the now placid ocean; thus was 
formed the clay-slate strata of the present era. By some 
internal causes submarine volcanoes again broke forth through 
the crust of the earth; the first eruption of this nature was 
apparently in about longitude 145°, and after successively 
extending the sphere of its action over the whole country, 
finally became extinguished beneath the ocean. The last of 
these convulsions, there is reason to surmise, occurred near 
Mount Benson, in South Australia. 
The most important result of this volcanic period, is the 
existence of the basalt, or as some English geologists term it, 
the trap formation. The rocks of this class are composed 
principally of felspar and hornblende, or augite, and when in 
a molten state, bear in many respects, a near resemblance to 
glass: hence the reason of the great frequency of the trap for- 
mation on our extensive plains, which were rapidly covered 
with the overflowing liquid mass. An enormous volume of 
steam generated in the interior of the globe, together with 
the constituent gases resulting from the actual decomposition 
of the sea water, were discharged through the crater, greatly 
assisting the volcanic power in ejecting the burning matter. 
Assuming the degree of heat necessary to liquify the basalt, 
to be identical with that of glass, we might thereby estimate 
the depth at which the former becomes fluid, in other words, 
ascertain the thickness of the solid crust of the earth. Now 
2650° Fahr. (the temperature at which glass liquifies) would 
require, (according to the annexed table, exhibiting the in- 
creased heat at the several depths named, and which are the 
more to be depended upon, as they are the results of actual 
borings taken for the purpose, by the Governments of Prussia 
and the Canton of Geneva, Switzerland), a depth of 105,000 
feet, or twenty miles nearly. This depth, which at first sight 
appears truly enormous, would scarcely equal the thickness 
of a coat of varnish on a globe of three feet in diameter.* 
* The Austrian Government caused twenty-seven borings to be made for 
the same purpose. No practical results could be expected from these deep and 
expensive sinkings; they were undertaken for the pure love of science, and 
afford an example which must excite shame in us for refusing to make a few 
borings in order to discover the real value of cnr coalfields. 
