58 The Causes and Phenomena of Earthquakes. 



authorities, and still giving allowance to the undoubted operation 

 of magnetic forces (connected as all physical agents are with one 

 another) and even admitting that the spots on the sun, owing to 

 their probable coincidences in relation to Aurora? and earthquakes 

 (which is a view recently made prominent) indicate an influence 

 upon the earth from without, yet it must be admitted that argu- 

 ments are in favour of an occasional access of water to a molten 

 mass below and to what is called " central heat," the steam thus 

 produced after the spheroidal state of the water has passed 

 rushing up and producing the blow. The vibrations preceding it 

 are considered as due to the spheroidal condition, and the after 

 vibrations as resulting from the motions impressed on the rocky 

 particles. 



The mathematician denies that these operations are limited 

 to Bischoff's assumed thickness of 20 or 30 miles, and 

 with this agrees a deduction (before mentioned) of Professor 

 Tyndall (who refers to Hopkins and Pairbairn), that pressure pro- 

 duces heat — quoting the former, who says that the deeper strata 

 would require a greater degree of heat to fuse them than the 

 upper strata ; and alleging that the heat of substances which 

 expand on solidifying is lowered by pressure. In the opposite 

 direction, this agency of hydrostatic pressure as producing earth- 

 quakes, has been advocated in a paper read before the Wer- 

 nerian Society (November 14th, 1840), by the Rev. J. Toplis, who 

 argues that the phenomena witnessed are generally explicable by 

 such an assumption. 



Lyell reminds us that the notions of the original fluidity of 

 the whole earth belonged to an age when geologists held opinions 

 that are now abjured as unsound. Granite was once considered 

 of the highest antiquity, and the so-called crystalline rocks were 

 held to be older than all the fossiliferous formations. W e now 

 know better, as I endeavoured to show in my paper, read before 

 this Society in 1865, " On the Transmutation of Rocks in Austra- 

 lasia. ,, The doctrine of metamorphism or transmutation explains 

 away a good deal of error respecting the formation of many rocks. 

 The exposure of such transmuted portions of the earth's crust 

 does not prove that the present crust rests on a universally con- 

 solidated base, but only that large tracts of upper beds have been 

 denuded, and that the slow processes of change during long epochs, 

 aided by water, have changed even fossiliferous deposits, which 

 occasionally cannot be distinguished in character from such 

 rocks as the older theorists believed to be the sole result of 

 igneous action. 



Mr. Hopkins argued that eruptions arise from the expansion of 

 gases in the molten matter, as proved by the constant ebullition 

 of Stromboli and Kirauea, and admits the evidence from sulphur 

 and other mineral matters condensed on the sides of volcanic 



