In respect to their mode of causation, earthquakes are usually divided into two classes, volcanic and tectonic, though it is often impossible to determine to which of the two classes a given earthquake should be referred. The volcanic earthquakes, which are closely associated in time and space with volcanic eruptions, are due to steam explosions and to the struggles of the rising lava within the earth to escape. In their typical manifestation, volcanic quakes have a definite centre of origin, which is in or near a volcano, and are rarely felt over any very extensive area of country. The earthquake of 1883 in the island of Ischia was of terrible violence and completely destroyed the town of Casamicciola, with great loss of life, yet the shock was hardly perceptible at Naples, a distance of twenty-two miles. The great eruption of Mauna Loa in the Hawaiian Islands in 1868 was preceded for six days by earthquakes of gradually increasing force, until they became frightfully destructive. When the lava burst out from the volcano, the earthquakes rapidly died away. Extreme as was the violence of these shocks, they were almost confined to the southern side of the island; elsewhere they did little damage, and at a distance of 150 miles were barely sensible.

The earthquakes of Central America and those at the base of Aetna. and Vesuvius and other volcanoes are of this class.

Tectonic earthquakes are believed to be due to stresses in the interior of the earth which, when suddenly yielded to by the rocks, cause the jar and shock which generates the earthquake. Macro-seisms are probably caused by the formation of new and great fractures and microseisms by settling along existing lines of fracture. Tectonic earthquakes are linear, the maximum destruc-tiveness being along the line of fracture and rapidly diminishing transversely to this line; owing to the deeper position of their origins and the greater masses of rock involved in the movement, their effects are far more widely spread than those of the volcanic class. Another distinction from the latter is the succession of after-shocks which follow the great tectonic quakes and which last longest away from the primary line of fracture. These are due to the gradual readjustment of the mosaic of blocks, which, as we have already seen, makes up the outermost part of the earth's crust.

Just how the internal stresses above referred to are generated, is by no means clear. The explanation usually accepted is that the earth is slowly contracting on account of the loss of heat, and that the crust, which must follow the shrinking interior, is being crowded into a smaller space, with resultant ruptures and shocks. However, this contractional hypothesis is altogether rejected by several high authorities, and no very satisfactory substitute for it has been proposed.

On the other hand, it is contended by some geologists that all earthquakes are essentially volcanic in origin. These observers call attention to the "marked synchronism or close following of the major disturbances, whether volcanic or seismic, at distantly removed points of the earth's surface, at certain periods." (Heil-prin.) It is undoubtedly true that such an association is indicated by many facts, but much remains to be learned before tip" full significance of these facts can be determined.