A Yorkshire Rector of the Eighteenth Century. 17 



the credit of having iirst suggested the potency of subterranean 

 aqueous vapour as a dynamical agent. A study of the Sills 

 or intrusive sheets, which are so abundant in the British Isles, 

 shows that the horizontal injection of the internal magma 

 (which owes its propulsive energy to the aqueous and other 

 vapours which it holds in solution under enormous pressure 

 and at a very high temperature) has sometimes taken place 

 on a gigantic scale. The Great Whin Sill of the North of 

 England, a mass of once molten rock, averaging 80 or 100 

 feet in thickness, has been injected between and across the 

 Carboniferous strata ov'er a distance of at least 80 miles, 

 possibly at successi\'e intervals of intrusion. Such enormous 

 inthrusts of the magma charged with its explosive vapour could 

 hardly fail to be accompanied by violent earthquakes. 



But in the case of most earthquakes it is not to this cause 

 that we must look for the explanation of their origin. Michell 

 undoubtedl}^ suggested a vera causa which may sometimes be 

 displayed in volcanic regions. He was probably influenced by 

 his familiarity with the little disturbed stratified formations 

 of the south of England, when drawing his analogy from the 

 air wave under the uplifted carpet, he conceived that vapour 

 suddenly generated at some depth from the surface of the 

 earth would force its way for a long distance between the planes 

 of stratification. But the crust of the earth is not everywhere 

 so regularly arranged as in the southern counties. Over wide 

 tracts it is much more complicated and almost everywhere it 

 is dislocated by faults by which strata, otherwise undisturbed, 

 have their continuity seriously interrupted. 



Much thought has been bestowed on the study of earth- 

 quakes since Michell's time, and a much clearer conception 

 has been formed of their cause and their phenomena. The 

 terrestrial crust is subject to enormous stresses under which 

 it is apt here and there to give way An}' such snap or fracture 

 starts a series of elastic waves which travel outwards in all 

 directions and are felt as an earthquake at the surface. Prob- 

 ably the great majority of earthquakes are due to this cause. 



Michell recognised that earthquake shocks are propagated 

 in successive waves in progressively lessening amphtude 

 through the solid earth. He also pointed out how the centre 

 of disturbance could be determined : ist, by observation of 

 the different quarters from which the shock arrives at a number 

 of distant places, the common intersection of lines drawn in 

 these directions showing the position of the centre ; 2nd, by 

 noting the time of arrival of the shock at different places, and 

 3rd from the successive hours cf arrival of the great sea-wave. 

 Employing these methods he computed that the focus of the 

 Lisbon earthquake lay under the Atlantic Ocean at a distance 

 of a degree of a great circle from Lisbon, and a degree and a 

 half from Oporto. 



1918 Jan. 1. 



