Still more remarkable is the sudden advent of the great shocks. There were no preliminary tremors or rumbling sound, no animals showed signs of uneasiness and no birds fluttered screaming from trees or ground. The shock of 1828, says Covelli, "was announced by three powerful blows coming almost vertically, from below upwards;" and the same words apply equally well to the earthquakes of 1881 and 1883. The destruction of houses in every case was practically instantaneous, and coincident with the first vibration. In all respects, tectonic earthquakes differ widely from the Ischian shocks. The epicentres of successive earthquakes are rarely coincident, but show a distinct tendency to migration along certain lines; the decline in intensity outwards from the epicentre is nearly always very gradual, and therefore indicative of a comparatively deep-seated focus; they are almost invariably preceded either by a series of slight shocks and rumbling sounds, or, in an unstable district, by a marked increase in their frequency. Distinctions, so great as these are, evidently remove the Ischian shocks from the category of tectonic earthquakes. ORIGIN OF THE ISCHIAN EARTHQUAKES. On the other hand, the Ischian earthquakes possess several features which connect them closely with true volcanic earthquakes. 1. They originate beneath the northern slope of Epomeo--a volcano that we have no reason to consider absolutely extinct, but rather as one subject to eruptions at long intervals of time--in a region as yet unoccupied by parasitic craters, but having the same relation to the central cone of Epomeo as those in which the recent craters of Monte Rotaro, Montagnone and Cremate are situated. 2. In both the earthquakes of 1881 and 1883, the epicentre is an elongated band, the axis of which, if produced, would pass through the centre of the old crater of Epomeo. Along the line of this band, occur the fumaroles of Monte Cito and Ignazio Verde and the thermal springs of the Rita and Capitello. These facts, as Professor Mercalli suggests, lead us to believe that the foci of the earthquakes coincide with a radial fracture of the volcano, the course of which, as traced by him, is represented by the continuous line in Fig. 14.[27] 3. Except in their relations with actual eruptions, the Ischian earthquakes resemble closely the true volcanic earthquakes which from time to time shake the flanks of Etna. These are marked by great intensity of the shock at the centre of a comparatively small disturbed area, epicentres often elongated radially to the cone, frequent repetition with similar characters in the same districts; and as a rule they precede by a short interval, but sometimes accompany or follow, volcanic eruptions.[28] Two other phenomena may be referred to as probably indicating some connection between Ischian earthquakes and the structure and history of Epomeo.