438 



NATURE 



[Sept. 6, 1883 



tions of the loose alluvial tufas and ash of the plains and 

 valley bottoms, whereas the destruction of houses built 

 on the rock was terrible, thus reversing the well known 

 parable. 



Putting aside, however, the speculative side of the 

 question, let us look to facts. From a careful examination 

 of observed azimuths and angles of emergence all point to 

 a plate-shaped focus, whose strike extends in a line from 

 Fontana, just west of Menella, to near the beach at Lacco. 

 The plane of this fissure is probably roughly perpendicular 

 to the surface, but may slightly dip towards the east as 

 the isoseismals are slightly nearer on the eastern side of 

 the seismic vertical, which as a necessity is not represented 

 by a point, but a line on the surface. The rupturing of 

 this plate-hke fissure was apparently greatest at a point 

 nearly midway between its extremities. 



The remarkable fact that I observed in the earthquake 



of 1881 that at Fontana the shock was almost vertical 

 like that at Menella is again repeated ; for which I then 

 proposed as an explanation the conduction along a tube 

 or column of highly elastic trachyte, filling the old chimney 

 of Epomeo, whilst the surrounding districts were protected 

 by the less elastic mattress of subjacent tufa. On the 

 present occasion Fontana, as before, has all its houses 

 fissured in such a manner as to demonstrate a vertical 

 shock ; but besides there is another set of cracks which 

 show a north to south path for a wave at a low angle of 

 emergence. At St. Nicola part of the altar has rocked in 

 the same direction, and between these two localities a 

 rock has been ruptured and projected to the south. 

 Whatever may be the explanation accepted, the fact 

 remains as unique in seismological history. 



That Vesuvius did not, or only to a slight extent, 

 sympathise with the seismic movement in the island of 



up r -m A 



CH0TTA D£J.Gt TCffM 



Sketch Map of the Island of Ischia. Scale I to So.ooo. Isoseismals marked thus ; ancient eruptive centresanj craters in do. ted circles. 



n, mesoseismal area = total destruction ; b, first isi iseismal = many houses fallen, rest require rebuilding ; c, second isoseismal = severely fissured. The 

 third isoseismal d jes not seem to cut the land ; its li nits, therefore are unknown. .1 , fissure, or focal centre. 



Ischia is no proof against the volcanic origin of the 

 earthquake. Admitting the hypothesis of a seismic wave 

 traversing a large tract of volcanic matter underlying 

 Southern Europe, such a wave may produce very slight 

 variation at volcanic vents, but may yet be sufficient to 

 determine the extension by rupture of a fissure, where the 

 resistance of the rock, and the tension of the volcanic 

 matter near the point of extension, are nearly equal. In 

 a paper lately read before the GeologLal Society of 

 London I endeavoured to prove that the explosive violence 

 of lava is due to the assimilation or solution of water 

 taken up from the water-bearing strata it traverses in its 

 passage from the main source towards the surface. Under 

 such conditions we may have very violent phenomena 

 produced, locally, without any sympathy in neighbouring 

 volcanic vents ; which at the same time explains I believe 

 one volcano bursting out in a violent paroxysmal eruption, 

 whilst a near companion is in no way affected. Why 



therefore should an abortive paroxysm disturb a neigh- 

 bouring active vent, as in the present example ? 



In various points that I have examined the coast of the 

 island, there was no apparent change of level, nor did 

 any success attend endeavours to discover any signs of 

 depression of the surface. 



It has been stated that the sea receded, but I 

 could not obtain any confirmation of the fact. As the 

 steamer that lay at anchor in the roads felt the shock, 

 it may probably have been due to the disturbance of the 

 water by the earth waves. 



As far as my inquiries have gone, the first symptom 

 was a distant sound like that of a carriage, almost imme- 

 diately accompanied by a tremor, then a terrific explosion 

 shading off into a number of reports. Most people not in 

 the mesoseismal area felt first the "susultatorio" or ver- 

 tical movement, followed by the undulatory, or, more 

 properly, lateral motion. This, as is well known, is due 



