In conclusion, it does not yet seem clear that we can rely absolutely upon a single cause as productive of all the convulsions of the earth’s crust. Internal local heat, pent up gases, suffice for volcanic phenomena; but earthquakes present so many peculiar variations that it seems almost imperative to many men to admit, at least, the modifying influence of other agencies. But so long as these agencies appear to be quite as frequently the modified as the modifier, no laws concerning them can be announced. Hence, internal conditions are the only clearly identified factors so far. There is quite as much difference of opinion as to how far beneath the surface the shocks originate. Robert Mallet’s investigations have led him to believe the depth cannot be over thirty miles, and that seven or eight miles is the limit for most, and his views are those of most scientists. But a few others conclude that we cannot find molten matter and gases to produce the concussion at a less depth than seventy-eight miles. But, as their conclusions are based largely upon the idea that the melting point of minerals is raised uniformly with increasing pressure, their conclusions must be rejected as unreliable. The character of the motion is well known. Each point of the surface begins to move with the vibration first upwards, then away from the center of shock, then downward and backwards. Thus, each point describes a small ellipse, which is repeated with each wave of vibration. If the longer axis of the ellipse be vertical, the main force of the concussion is directed upwards; if the shorter one be upright, the shock is an undulatory one. An alternation of the two forms the most destructive combination. The difference is readily perceived in the effects produced. A sudden upward shock may wreck the roofs or floors of buildings, while an undulatory one brings down the walls. Houses erected on sand, immediately overlaying compact rock, usually suffer most during earthquakes. The effect is that of the vibration of a sheet of glass covered with sand. But, if a second sheet of glass be placed on that, the vibration is hardly communicated to it at all. So, while sand is a bad foundation, a sand-bed beneath the surface seems to deaden the shock. It is not difficult to understand that lofty buildings, and those of stone or brick, must be vastly more dangerous than those of wood, and low and broad. Throughout many portions of Central and South America, the people endeavor to compromise, by building houses of stone, but low and massive, with very light roofs. These are far less safe than light structures of wood; also, it is clear that cupolas and towers must be peculiarly liable to injury. For this reason, churches have often suffered more from shocks than other buildings, and the throngs of penitents who flock to them in the hope of propitiating an offended providence are often the first victims of an earthquake.