SIMPLE DIRECT PRESSURE. 51 



are not infrequent, nor js it very unusual to find such a pair of systems of 

 fissures accompanied by a second similar pair in a plane at right angles 

 to the first. The residual blocks are then bounded by from four to eight 

 planes. In the last case four of the planes are parallel to the other four * 



In many cases the rock does not rupture without previous deforma- 

 tion of considerable amount. When this happens the lines of rupture 

 make an angle of nTore than 45° with the line of force. The normal to 

 the direction of rupture then makes an angle w with the line of force, 

 and this angle decreases with the deformation. If the deformation were 

 very great,as it would be with a mass of india-rubber, u> would approach 

 zero. If the direct pressure were relieved by rupture and the rock were 

 perfectly elastic, the residual fragments would recover their original 

 shape, and their acute angles would then lie in the line offeree. 



Thus when rocks show fissures cutting one another at acute angles 

 it is certain that finite deformation has taken place. If the mass has 

 remained under tension, the line of force when direct bisects the obtuse 

 angles. If the mass has been relieved of pressure and the rocks have 

 acted as elastic masses, the line of force bisects the acute angles. 



It is usually possible from general conditions to judge which of two 

 rectangular directions is the more probably that from which a rupturing 

 force has acted. I have, however, never yet met an instance in which it 

 seemed to me that the line of force bisected the acute angles" of fissure 

 systems. Orogenic forces are commonly very persistent, and even if a 

 mass behaved as substantially elastic up to the moment of rupture, it is 

 improbable that the residual blocks would continue capable of regaining 

 their original shape after the lapse of, say, even a few years. In many 

 cases it is quite clear that deformation has become permanent. Thus I 

 have examined very numerous pebbles in conglomerates, some of which 

 had been much flattened by pressure and others also much fractured. 

 The direction of flattening was then a certain indication of the direction 

 of force, and this direction bisected the obtuse angles between the fissure 

 systems intersecting the pebbles. In other cases the character of slicken- 

 sides and accompanying faults shows that no reversal of motion has 

 taken place, and that the residual masses must have lost the elasticity 

 which they seem to have exhibited up to the moment of rupture. 



Observations on artificial structures seem to confirm this opinion. It 

 lias been pointed out by Mr. Clarence; King aim 1 others that slabs of 

 marble supported at their ends or corners gradually sag toward the 

 center. So, too, in old buildings, such as the Alhamhra, 1 have seen 

 slabs of rock very much bent by c\m\ pressures acting for hundreds of 



* When a rock fragment is lioun.li> I by planes \\ ith more than lour differently directed normals, 

 it must have undergone successive ruptures 



