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 Enghsh 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 hquid 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 sohd crust of the earth. Now 

 2650*^ Tahr. (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 

 borino-s 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 onr coalfields. 



