Fresh Notes on the Crater Linne. 219 



logists imagine to exist beneath the solid crust of our planet. 

 The depth of earthquake movements, or rather, of the focus of 

 earthquake disturbance, has been investigated by Mr. Mallet, 

 in his work on the Neapolitan earthquake of 1857,* and from 

 mathematical considerations, applied to actual observations, 

 he arrived at the conclusion, ""that out of twenty- six separate 

 wave paths, twenty-three started from the seismic vertical, at a 

 depth of about 1\ geographical miles, or of 43,284 feet. The 

 maximum depth is 8-J- geographical miles, or 49,359 feet, and 

 the minimum depth is 2f geographical miles, or 16,705 feet. 

 Eighteen of the wave paths start from the seismic vertical 

 within a vertical range in depth of 12,000 feet, and having a 

 common focal depth of 5^- geographical miles, or of 34,930 feet, 

 which may be taken as the depth of the focus. The extreme 

 vertical range between maximum and minimum depth is 32,654 

 feet. On examining the diagram, however, having regard to 

 the points in the seismic vertical where the wave paths start 

 thickest, it will be apparent that the probable vertical depth of 

 the focal cavity itself does not exceed 3 geographical miles, or 

 18,225 feet at the outside/' 



Applyiug the same principle of calculation to the larger 

 earthquake phenomena of South America, he considers that 

 the greatest probable depth of earthquake action is somewhat 

 less than thirty-one geographical miles, and, ' c therefore, only 

 just touches the depth which upon received notions, as to the 

 increment of hypogeal temperature, is supposed to form the 

 upper surface of the imaginary ocean of liquid lava of the earth's 

 interior." 



The connexion between earthquakes and volcanoes is gene- 

 rally admitted, and though earthquakes frequently occur at 

 considerable distances from volcanoes, an eruption of the. latter 

 usually acts as a safety valve, and lets out highly-heated materials 

 which would otherwise, by their efforts of expansion, produce 

 tremendous shocks. Yolcanoes, like earthquakes, may result 

 from chemical actions at the moderate depths spoken of by Mr. 

 Mallet, and if this is the case on the earth, it may be so like- 

 wise on the moon. 



On the earth, the magnitude of volcanic forces, whether 

 exhibited in eruptions or earthquakes, seems to stand in fre- 

 quent, if not constant relation to the height of mountain chains, 

 and the depths of the foci of disturbance probably vary consider- 

 ably, as Mr. Mallet suggests. If we apply this principle to the 

 moon, Linne would be connected with the volcanic district of 

 the Caucasus and the Apennines, which are amongst the high- 

 est of the lunar chains, the former reaching an extreme elevation 

 of 17,138 Paris feet, and the latter of 16,934 Paris feet, or onc- 



* First Principles of Observational Seismology. — Chapman and Hall. 



