Page:Popular Science Monthly Volume 27.djvu/847

Rh the terrestrial crust. The French Academicians Bouguer and La Condamine had already come to a similar conclusion in 1741, when they were measuring zenith distances of stars. It could hardly have been anticipated that the observation of the stars would reveal processes that were going on down in the interior of our planet.

These movements, not directly perceptible by our senses, are subjected to an attentive daily study in Italy, at twenty-eight stations, scattered from one end of the peninsula to the other; and the results of the observations are centralized at the geodynamic observatory in Rome under the direction of M. Rossi. The movements are distinguished as very rapid and prolonged tremors (tremiti), and microseismic undulations, characterized by their extreme slowness. The observations of each day are depicted on a map of Italy by means of conventional signs, so that they may be followed in all their details as well as in the aggregate, at a single glance. Weak as these phenomena may be, they are well worthy of attention by reason of their continuous and general character; for they reveal an internal labor under the foundations of the ground, that never stops.

The crust of the earth also suffers displacements of a secular slowness, unaccompanied by any sudden movement; a class of phenomena which would never have been made known, if the mean level of the sea did not offer, at the shore, an invariable beach-mark by which to measure them. Tracts, which have manifestly been submerged within historical times, are now above the level of the sea, and constitute what are called raised beaches; while, on the other hand, forests, described in history as partly submarine, are now, in consequence of the depression of the soil, wholly under water. Such changes of level, very numerous and well established in all parts of the globe, are sometimes repeated in an oscillatory fashion of alternate elevations and depressions. They were formerly attributed to changes in the level of the sea, but the movement is in the land. They are in continuance of analogous changes which took place on vast scales during all the ancient geological periods. They are not to be confounded with superficial erosions and delta formations, which are quite different in character as well as in cause.

In view of these facts we are justified in saying that the crust of the earth is very far from being still. At every instant and in many of its parts it is undergoing very pronounced and often violent shocks. More frequently the movements are simply thrills, which can be discovered and studied only by a kind of auscultation. They are really continuous and of different kinds. It remains to inquire to what subterranean causes they should be attributed.

Numerous as the observations on earthquakes maybe, they concern merely the external manifestations of a phenomenon the source of which is completely hidden from us, separated by a considerable thickness of rocks. Hence we have no clear, certain data by which to