1876.] 283 [Niles. 



and displacements of rock, attended by more violent movements, 

 assuming the form of genuine but local earthquakes. Thus the ero- 

 sive action of streams, deepening valleys and forming gorges, 1 may in 

 part account for the frequency of minor earthquakes in valley regions. 

 So also the increased amount of moisture in the rocks in wet seasons 

 and the expanding energy of the frosts of winter, may furnish suffi- 

 cient assistance to enable the power to overcome the strength of the 

 rock material, and so precipitate the violent movements as to be the 

 occasion of the increased number of earthquakes observed in winter 

 and during wet seasons. 



In excavating hills and mountains for railroad and other construc- 

 tions, explosions have sometimes occurred which could not be ac- 

 counted for as the results of any artificial power. I would call 

 attention to the evidences of a lateral compression as a probable 

 explanation of such phenomena. I would also suggest that some of 

 those explosions which some have supposed might have been caused 

 by the oxydation of pyrites or other changes, may have been pro- 

 duced by the yielding of the rock to the force under consideration. 

 Also strange sounds in the earth have frequently been so candidly 

 and intelligently reported as not to be satisfactorily rejected on the 

 supposition of fear, superstition or imagination. I would therefore 

 suggest the possibility of some of these noises being the result of the 

 the more gradual movements of rock, such as have been observed 

 at the quarries above described. 



Last September I was assisted by Mr. Silas W. Holman in making 

 some careful measurements of a portion of a bed at Monson, which 

 by expansion had formed an anticlinal arch without being broken at 

 any point. From base to base the arch measured fifty-nine feet and 

 nine inches. The thickness of the bed varied from ten to sixteen 

 inches. Although after our measurements were taken the stone ex- 

 panded still more before breaking, yet the amount of expansion 

 a.t that time was more than one thousandth of the original length 

 of the stone. If a thousand miles of rock were subjected to the same 

 compression throughout, and then permitted to expand as this did, 

 there would be an increase of on e mile in its lateral extent. Mr. Hol- 

 man has calculated that if one thousand miles of rock were to expand 

 throughout its length in this proportion, causing thereby an eleva- 

 tion of the mass in the form of an arc of a circle, the original one 

 thousand miles being the chord, the elevation of the centre of the 

 1 G. H. Otto Volger. Petermann's Geogr. Mittheilungen, 1856, Heft III, 



