Narrative of the surveying voyages of his majesty's ships Adventure and Beagle (vol.3)

March, 1835. direction of vibration. 375 other. Captain FitzRoy* has Ukewise remarked, that the greater number of the masses of brickwork were thrown down towards the N.E. Both these circumstances per- fectly agree with the general idea of the undulation having come from the S.W. ; in which quarter subterranean noises were also sometimes heard. It is evident on this suppo- sition, that the N.W. and S. E. walls, being nearly coin- cident with the hne of undulation (or wdth the crests of the successive waves), would be much more likely to fall than those which had their extremities presented towards the point whence the vibration proceeded ; for, in the first case, the whole wall would be thrown at the same moment out of its perpendicular. This may be illustrated by placing books edgewise on a carpet, and then, after the manner suggested by Michell, imitating the undulations of an earthquake : it will be found, that they fall with more or less readiness, according to their direction. The fissures in the ground, though not uniform, generally had a S.E. and N.W. direction 5 1 and therefore they corresponded to the lines of principal flexure. Bearing in mind all these cir- cumstances, which so clearly point to the S.W. as the chief focus of disturbance, it is a very interesting fact that the island of S. Maria,t situated in that quarter, was during the general uplifting of the land (to which I shall presently refer) raised to nearly three times the altitude of any other part of the coast. The different resistance offered by the walls, according to their direction, was well exemplified in the case of the cathe- dral. The side which fronted the N.E. presented a grand pile of ruins, in the midst of which door-cases and masses of timber stood up, as if floating in a stream. Some of the angular blocks of brickwork were of great dimensions ; and they had been rolled to a distance on the level plaza, like fragments of rock round the base of some high moun- * Sketch of Surveying Voyages of Adventure and Beagle by Captain FitzRoy, Royal Geography Journal, vol. vi., p. 320. f Ditto, p. 327, ft j)assim.