410 T. If. Recede — Cause of Active Compressive Stress 



and that is the greater development of jointing in the surface 

 rocks. Joints are widened by atmospheric agencies so that the 

 surface rocks are frequently cut up into blocks not in actual 

 contact. This is very noticeable in the sandstone quarries in 

 Darley Dale, Derbyshire, where the joints are sometimes filled 

 with infiltrated bowlder clay. 



It is evident that if the surface rocks are not continuous over 

 a considerable area the throwing up of the anticlinal ridge 

 could not take place because there could be no accumulation of 

 lateral stress. 



There is no doubt that alternations of temperature take place 

 in deep seated rocks, causing their bulk to undergo considera- 

 ble variation. These rocks, owing to the pressure they are 

 subjected to, are in absolute contact and form a homogeneous 

 mass. I have attempted to show (Origin of Mountain Ranges, 

 Chap. XXV), that the sudden release of these accumulated 

 stresses, whether of tension or compression in deep seated 

 rocks, is the cause of earthquakes that happen away from vol- 

 canic centers, and also that earthquakes are more frequent and 

 greater in intensity in areas occupied by the newer rocks such 

 as the Tertiary in all parts of the globe. It is in these areas 

 that the greatest underground fluctuations of temperature occur 

 in the crust of the earth. While my work was going through. 

 the press, the earthquake that ravaged Georgia and South 

 Carolina, known as the Charleston earthquake, occurred on the 

 night of August 31, 1886, thus emphasizing in an nnfortunate 

 way the principles I had expounded. The substratum here is 

 of Tertiary rocks and the area was not recognized as one of 

 great seismic activity. On the contrary an eminent geologist 

 had only just before been insisting upon the great stability of 

 the Atlantic borders of the JSTorth American continent. 



These changes of temperature cannot, however, excepting in 

 a secondary manner affect surface rocks, for they are kept by 

 atmospheric influences at the mean temperature of the station 

 at which they occur ; we may therefore dismiss change of tem- 

 perature of the surface rocks themselves as a vera causa. 



The uplift on the Lower Fox River, though a striking exam- 

 ple, can be paralleled by other instances of lateral pressure in 

 surface rocks disclosed in the process of quarrying, suggesting 

 a similar release of accumulated stress by unloading. Professor 

 Kenny Hughes gives an instance (G-eol. Mag., 1887, p. 511) of 

 the bursting up of the floor of a limestone quarry at Dent Head 

 and also of the floor of a tunnel at Ribble Head in Yorkshire, 

 but he attributes these, whether rightly or not I am unable to 

 say, to the fact that the beds rest on shale, inferring that the 

 unequal pressure caused by the removal of part of the rock 



