34 TRANSACTIONS OF THE [OCT. 18, 



to make new series of sedimentary deposits. As these accumu- 

 late they not only impose new burdens on the underlying rocks, 

 but by acting as blankets and preventing the escape of heat, they 

 promote the softening and weakening of a belt of sea bottom. 

 This process has produced great changes in the surface topo- 

 graphy of many continents, and it is credited with the forma- 

 tion of a number of littoral mountain chains. The blanketed 

 belt of off-shore sea bottom, softened by heat, yields to lateral 

 pressure, and is forced up in a series of faults and folds. There 

 is little doubt that the loading of the sea bottom with the pro- 

 ducts of erosion has been one cause of the earthquake vibrations 

 which have been so frequent along our Atlantic coast. 



Periodicity of Eartliquake^. — Very naturally an effort has 

 been made to connect earthquakes with the changing relations 

 of the sun and moon. M, Perrey, of Dijon, France, has tabu- 

 lated the records of 2,225 earthquakes which occurred between 

 the years 306 and 1845. Of these he found that 1,712 took place 

 in winter and spring, and 1,335 in summer and autumn. By Mr. 

 Eobert Mallett between 6,000 and 7,000 earthquake shocks are 

 enumerated as having taken place in Europe onl3^ Judging 

 from all these it seems that earthquakes are a little more fre- 

 quent when the attractions of the sun and moon are combined 

 or opposed — that is at new and full moon — and when the earth 

 is nearest the sun. 



These data, although still defective, tend to support the theory 

 of the fluidity of the interior of the earth, and confirm the tes- 

 timony of volcanoes and of the secular oscillations of the earth's 

 crust. 



Areas of Exemption. — Probably no part of the earth's surface 

 has been always free from earthquakes. Nevada, Utah, New 

 Mexico and Arizona were in Tertiary times more completely 

 broken up and devastated by earthquakes and volcanoes than any 

 other country known to us, but in that same region profound 

 peace prevailed from the Cambrian to the Cretaceous ages, many 

 millions of years. Since the Tertiary, the Colorado plateau has 

 been remarkable for its stability. This is shown by the sand- 

 stone pillars several hundred feet high standing at the mouth of 

 the Canon of Chelly, and in the Colorado valley near the junc- 

 tion of the Grand and Green Eivers. These columns have been 

 formed by the slow removal of the material around them, a 

 work of ages; and earthquakes would have brought them down 

 in ruins, as they have shattered the monuments of Baalbeck, 

 Tanis and Karnak. 



Bibliography. — " First Principles of Observational Seis- 

 mology," Robert Mallett; ^* The Facts and Theory of Earthquake 

 Phenomena," R. Mallett, Reports of British Association, 1850- 



