To this testimony little is added by the narrative of Dion Cassius, which was written a century and a half later and is known to us only in abstract; Dion dwells at greater length on the powerful impression which the terrible convulsion of nature left upon those who were living at that time. With the help of the letters of Pliny, in connection with the facts established by the excavations, it is possible to picture to ourselves the progress of the eruption with a fair degree of clearness. The subterranean fires of Vesuvius pressed upward to find an outlet. The accumulations of volcanic dust and pumice stone that had been heaped up on the mountain by former eruptions were again hurled to a great height, and came down upon the surrounding country. On the west side of Vesuvius they mingled with torrents of rain, and flowed as a vast stream of mud down over Herculaneum. On the south side, driven by a northwest wind as they descended from the upper air, they spread out into a thick cloud, which covered Pompeii and the plain of the Sarno. Out of this cloud first broken fragments of pumice stone--the average size not larger than a walnut--rained down to the depth of eight to ten feet; then followed volcanic dust, wet as it fell by a downpour of water, to the depth of six or seven feet. With the storm of dust came successive shocks of earthquake. Such was, in outline, the course of the eruption. It must have begun early in the morning of the twenty-fourth, and the stream of mud must have commenced immediately to move in the direction of Herculaneum; for shortly after one o'clock on that day the admiral Pliny at Misenum received letters from the region threatened, saying that the danger was imminent, and that escape was possible only by sea. Even then the Younger Pliny saw, high above Vesuvius, the cloud, shaped like an umbrella pine, which was to rain down destruction on Pompeii. Toward evening, the ships off Herculaneum ran into the hail of pumice stone, which, during the night, reached Stabiae and so increased in violence that the admiral Pliny was obliged to leave his sleeping room from fear that the door would be blocked up by the falling masses. Early in the morning of the twenty-fifth there was a severe shock of earthquake, which was felt as far as Misenum. Then the dust began to fall, and a cloud of fearful blackness, pierced through and through with flashes of lightning, settled down over land and sea. At Misenum, even, it became dark; "not," says Pliny, "as on a cloudy night when there is no moon, but as in a room which has been completely closed."