Page:Great Neapolitan Earthquake of 1857.djvu/444

354 cubed, contained fully 35,000 cubic feet, and weighed about 2,500 tons. The surface of fracture, of which about $2⁄5$ths was formed of planes of separation (transverse joints) in the rock, and the remainder through the solid stone, was an irregular circular figure, and about 20 ft. × 18 ft., the plane of fracture generally being inclined about 45° to the horizon.

The rock was very soft, arenaceous, and chalky limestone, perfectly white, and a hand specimen, when fresh fractured, could, like sugar, be rubbed away against another piece; so that the cohesive energy was not great, and the inertia of so great and high a mass was quite sufficient, at a very moderate velocity, to bring it over. It however has a peculiar interest, as the first example I have seen of actual rock fracture by the direct operation of the shock.

Neither the position of the centre of gravity, nor of that of resistance of the fractured base could be fixed with sufficient accuracy to enable any calculation to be based upon the fracture and fall of the mass, that would give a trustworthy measure of velocity or of direction.

On examining the buildings in the town on the south and south-west slopes, where the damage done was greatest, I found evidence by fissures and projected wedges of masonry, giving a direction of wave-path varying between the limits of 155° E. of north to 167° E. of north, and angles of emergence (all, however, proving emergence from the north and N.W.) varying between 20° and 25°. Some of the fractures from which the latter elements are taken are visible in the Photog. No. 210, though not so in its lithographic reproduction.

Many circumstances indicated the transit of two shocks crossing obliquely, such as the twisting of objects on their