Niles.] 282 [April 5, 



Furthermore, although we may not at this time be able to trace a 

 direct connection with them, yet the recent changes of level of the 

 Atlantic Coast seem to depend upon an activity in the same north 

 and south direction. From northern Greenland to Florida we find 

 extents of rising coast alternating with others that are subsiding. 

 We can better understand how these alternate areas of movement 

 could be produced by the slight foldings, resulting from this north 

 and south compression, than from any lateral pressure acting per- 

 pendicularly to the trend of the coast line. 



It will be noticed that the more violent rendings and displacements 

 of rock at Monson and Berea are similar to small earthquakes in 

 their general characteristics. Many well known facts have led us to 

 suppose that at least some of the slight earthquake shocks of this 

 and other non-volcanic regions are caused by sudden and often loca 

 displacements of the rock-masses which are near the surface. My 

 observations at the quarries above mentioned teach me to look for 

 like phenomena where the rocks are in distinct and continuous lay- 

 ers which are not firmly united together. Where the rocks are much 

 divided by open joints, or are otherwise broken, the force would have 

 little or no opportunity for manifestation. We have seen that at the 

 localities studied the beds of rock appear to be compressed to nearly 

 the extent of their strength for resistance, and that if the power 

 becomes concentrated, or is slightly assisted, the layers are flexed or 

 broken, and the more violent actions are sometimes produced. These 

 and other associated facts demonstrate, I believe, the continual 

 existence of a force fully adequate for the production of certain 

 earthquake phenomena. If we accept this deduction, we may then 

 conclude that such movements as are referred to here may often be 

 caused, not by the sudden introduction or by the awakening of some 

 subterranean power, but by the yielding of the rock-masses to that 

 lateral compression to which they are continually subjected. 



If this be true, the cause of a certain class of earthquake phenom- 

 ena is an ever present one, only requiring favorable occasions for the 

 manifestation of its power. It having been found that the artificial 

 removal of comparatively small amounts of stone has caused such 

 concentrations of this power that the adjoining rocks have been 

 shaken and rent, we may reasonably expect that the much more 

 extensive excavation of the strata by the natural processes of de- 

 nudation would cause a still greater concentration of this force, 

 and would thus give rise to similar but more extensive yieldings 



