A STUDY OF RECENT EARTHQUAKES. 825 



the terrestrial crust. The French Academicians Bouguer and La Con- 

 damine 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 sub- 

 jected 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 obseiwations are centralized at the geodynamic observatory in 

 Rome under the direction of M. Rossi. The movements are distin- 

 guished as very rapid and prolonged tremors (fremiti), and microseis- 

 mic undulations, characterized by their extreme slowness. The ob- 

 servations 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 phenome- 

 na may be, they are well worthy of attention by reason of their con- 

 tinuous 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 slow- 

 ness, 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 with- 

 in historical times, are now above the level of the sea, and consti- 

 tute 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 some- 

 times repeated in an oscillatory fashion of alternate elevations and de- 

 pressions. 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 an- 

 cient geological periods. They are not to be confounded with superfi- 

 cial erosions and delta formations, which are quite different in charac- 

 ter 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 dis- 

 covered and studied only by a kind of auscultation. They are really 

 continuous and of different kinds. It remains to inquire to what sub- 

 terranean 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 thick- 

 ness of rocks. Hence we have no clear, certain data by which to sup- 



