80 Proceedings of the Royal Society of Victoria. 



It may I think also be concluded, from a com])avi.son of the 

 two beds of lava, the one referred to and the other close to 

 the present cone, that the internal heat at Tana is diminishing. 

 It may reasonably be assumed that the lava flow near 

 Mount Yasur is the most recent, and this from the way in 

 Avhich it stands piled up presents the appearance of having 

 issued forth in a much more viscous state than that on the 

 outer coast-line. 



With regard to the depth below the surface at which the 

 force causing the eruption is generated, two circumstances 

 will assist in determining this approximately. The first is 

 the accompanying earthquake, the second the area of surface 

 upheaved in the violent shocks. Every erui)tion is 

 attended by an earthquake, which varies in force precisely 

 as the eruption varies in magnitude, and they occur always 

 at the same instant ; these earthquakes are confined to the 

 volcanic district, the most severe ever felt, that of 1878, not 

 being noticeable 12 miles awa3^ The origin, therefore, could 

 not have been deep-seated, it being very evident that the 

 deeper the oi'igin, the more widespread and uniform would 

 be the result. In the absence of accurate data as to the 

 direction, the angle of emergence, and the relative force at 

 diflerent distances from the centre, no precise estimate can 

 be made. The depth of the focus of an earthquake which 

 shook the whole of Italy was fixed by Mr. Mallet as being 

 about six or seven miles. The origin of the Tana earthquakes 

 and eruptions must therefore be very much less than this. 

 The fact also of the area upheaved or bulged upwards 

 during the eruptions of 1878 and afterwards, being so small, 

 would a])pear to strengthen the view that the origin of the 

 disturbance is not deep-seated, for it is quite inipobsi1)le to 

 conceive of a small portion a few hundred yards in extent 

 being lifted, the land around remaining in situ, without 

 bringing the generating force very close indeed. 



The connection of volcanic eruptions with the atmospheric 

 pressure has frequently, and especially in late years, received 

 attention, without however establishing it as a matter of 

 fact. This no doubt is due to the incompleteness of the 

 observations, and to the fact that only in connection with 

 such volcanoes as Stromboli, and that on Tana, wliere the 

 eruptions are so frequent, is it possible to take a sufficient 

 nunibei' of observations to be of any value in tiiis matter. 



Professor Judd, the latest waiter on this subject, referring 

 to Stromboli, says : — " Whether the ])opular idea, that 



