VULCANOLOGY AND SEISMOLOGY. 221 



Mississippi Valley, and 147 ou the Atlantic slope, giving an average 

 frequency of about one every twelve days for the whole region, and 

 once a month for the Atlantic slope. {Science, iv, 569.) 



On July 28, 1883, portions of the island of Ischia were devastated by 

 a severe earthquake, which has given rise to a considerable amount of 

 literature. The following account is condensed from a variety of sources 

 mentioned in the bibliography : 



Ischia contains about 26 square miles, and in the center of the island 

 rises Mount Epomeo, 792 meters high, an old volcano, which during 

 historic times has shown its activity only by hot springs and earth- 

 quakes. The thermal springs, with the pleasant climate, have made 

 Ischia a favorite summer resort, and during the season the town of Cas- 

 amicciola is usually crowded with strangers. There had been some pre- 

 vious indications of unusual subterranean activity — some hot springs 

 had shown abnormal variations of temperature, some slight earthquakes 

 were felt, and the instruments in the seisraological observatories at 

 Naples and Eome were in increased motion; but nothing gave any 

 warning of where the blow would strike. The violent shock came about 

 Qh 25ni p^ ju_ Qu Saturday, July 28, and the greatest damage was done 

 at Casamicciola and vicinity. This town, built ou two small hills on 

 the north slope of Ejiomeo, was entirely destroyed. A performance 

 was in progress at the theater, and when the building collapsed at the 

 shock many persons were buried in the ruins. Lacco Ameno, on the 

 coast northwest of Casamicciola, was also mostly destroj'ed, and Forio 

 was much damaged. The town of Ischia, ou the east coast, was severely 

 shaken, without suifering serious harm. But the villages of Fontana 

 and Serrara, situated in the interior of the island, and indeed within 

 the old crater, were great sufferers, as was also Barano. Two large land- 

 slii^s were caused on the north slope of Epomeo, but no true fissures 

 were found anywhere and no apparent changes of level. The num- 

 ber of casualties in the island, as stated by the otficial commission of 

 the Italian Government, was 2,313 killed and 762 wounded. In Casa- 

 micciola there were before the shock 672 dwellings and 4,300 inhabi- 

 tants, of which 537 dwellings were destroyed entirely and 1,784 i)eopIe 

 were killed. 



In investigating the origin of this earthquake, L. Baldacci {Boll, del 

 B. Com. Geol. d^Italia, iv, 157) traces two lines of hot springs, one east 

 and west, the other north-northwest to south-southeast, which he re- 

 gards as indicating the position of old cleavage lines intersecting at Casa- 

 micciola, and to which he attributes this earthquake. H. J. Johnston- 

 Lavis {Nature, xxviii, 437) has drawn isoseismals about the focus, 

 finding them to be elongated ellipses whose minor axes are nearly in 

 the line of the north and south fissure just mentioned, and concludes 

 that they indicate a plate- shaped focus, whose strike extends in a line 

 from Fontana to near the beach at Lacco. . C. G. Kockwood finds the 

 cause in a rupture taking place along an old volcanic fissure directed 



