TECTONIC DEFORMATION 789 



layers one within the otlier. In the beds they are seen to consist of 

 two sets interlocking from opposite sides, the top and bottom of the 

 beds afifected being formed by the bases of the two sets of cones. 



Professor Marsh ( 16) has suggested that this structure is the 

 result of concretions forming under pressure. It has also been 

 held that crystallization of the calcium carbonate in the bed is re- 

 sponsible for the formation of this structure. 



B. Tectonic or Orogenic Deformations. 



Of late the term tectonic has come to be more specially applied 

 in geology to larger rock structures, due to disturbances such as ac- 

 company and bring about the formation of mountains. In other 

 words, orogenic disturbances have come to be considered the chief 

 causes in the production of tectonic features. Lateral pressure 

 seems to be the chief active agent responsible for such deformation, 

 but it is by no means certain that the products of such lateral com- 

 pression are always distinguishable from those of vertical compres- 

 sion by the superincumbent strata. 



The various types of tectonic deformation will be considered, be- 

 ginning with tectonic jointing. 



d. Deformation Resulting in Fractures and Related Structures. 



12. Joints. Rocks separated into more or less regular blocks by 

 natural fissures are said to be jointed. Joints may be due to shrink- 

 age on cooling, or on desiccation, forming shrinkage joints, already 

 discussed, or they may be caused by folding or other tensile stresses, 

 giving tension joints, or finally they may be due to compression on 

 folding, forming compression joints. Both tension and compression 

 joints frequently occur together. In tension joints the surfaces will 

 be rough if the rock is arenaceous or rudaceous and the grains 

 weakly held. If the rock is lutaceous or otherwise a strong, tolera- 

 bly homogeneous rock, the fracture will be smooth and sharply cut. 

 Faulting and slickensiding may develop along such joints when sub- 

 jected to pressure, and deposit of mineral matter will take place 

 along the joint surfaces. 



Daubree (3) has shown by experiments with glass plates that, 

 if a brittle stratum is subjected to torsion, when the limit of elas- 

 ticity is reached it will break, with two sets of parallel fractures 

 forming nearly a right angle with each other (Figs. 174, 175). 



