52 G. K. Gilbert — Origin of Jointed Structure. 



parting planes are separated by wide interspaces, while in the 

 case of slaty cleavage the interspaces are small. Another 

 difference is exhibited by the internal structures of the blocks 

 separated by the division planes. In slaty structure the blocks 

 are themselves cleavable in the direction parallel to the planes 

 of division, while in typical jointed structure the blocks are 

 not thus cleavable. These two differences are perfectly evi- 

 dent when typical samples are considered, but it may be 

 doubted if they are universal and diagnostic. Many example- 

 are difficult to class with one type or the other, and there ap- 

 pears to be a gradation of character alike in regard to the 

 thickness of the parted blocks and in regard to their cleava* 

 bility. It is practically impossible to draw a hard and fast 

 line between the two structures, and the geologists who regard 

 jointed structure as identical with slaty cleavage certainly have 

 much to say in support of their opinion. 



When however we come to the question of cause, serious 

 doubts arise. We have a well-sustained theory that slaty 

 cleavage is due to compression, the direction of the compress- 

 ing force being normal to the planes of cleavage. If this 

 theory is true, and if joints are essentially identical with 

 cleavage planes, then joints too are caused by pressure and 

 compression. Should we grant this we must accept joints in 

 every instance as evidence of compression, and we must con- 

 elude that all or nearly all level-kin- strata have been sub- 

 jected to coercive pressure from one or more directions. Two 

 systems of joints must indicate pressure from two directions at 

 rent times, for coincident pressures could only induce 

 jointing planes normal to the direction of their mechanical re- 

 sultant. The occurrence in the same rock-mass of a number 

 of systems of joints, theoretically indicates the successive, and 

 not the coincident, existence of the same number of mechanical 

 stresses. The application of this rule to the Quaternary strata 

 of the Salt Lake Desert imposes a severe strain upon the 

 ion, for it requires us to believe that a broad sheet of 

 freshly -formed sediments— so fresh that the shore-trace of the 

 formative lake has scarcely been impaired by the weather- 

 has been compressed by forces acting first in one direction and 

 afterward in a direction nearly at right angles. There are, in- 

 deed, evidences of Post- Quaternary orographic movements in 

 the region, but those movements were small and vertical, and 

 the type of structure exhibited by all the surrounding mountains 

 is one implying vertical displacement and no lateral contrac- 



If the theory of lateral compression were valid, and if each 

 epoch of compression left its record in a system of joints, it 

 would be reasonable to expect that the older strata would be 



