820 PRINCIPLES OF STRATIGRAPHY 



Downward probably most faults pass into flexures, these flexures 

 dying out at still greater depth. Van Hise thinks that 5,000 meters 

 is a possible depth, at which important faults disappear, though 

 some may extend to the depth of a number of miles. Others, how- 

 ever, regard the necessary depth as very much less. 



C. Contact Deformations. 



Under this heading may be placed changes in the rock mass as 

 a whole, produced by contact with a deforming agent. The deform- 

 ing force is heat or cold, and the agents conveying the former are 

 igneous masses (intruded or extruded), hot waters or gases, and 

 the direct rays of the sun. Heating rock masses by any of these 

 agents results in expansion of the rock. The agents conveying cold 

 are glaciers and the cold atmosphere. Their action on the rocks 

 results in contraction. The chief structures produced by these 

 agents singly or in conjunction are prismatic jointing and insola- 

 tion joints. 



21. Prismatic Jointing Due to Contact zvith Igneous Masses. 

 When igneous masses come in contact with sedimentary rocks a 

 prismatic structure is not infrequently developed. This has been 

 noted in clays, marls, sandstones, brown coal, seam coal and even 

 in dolomites. Beautiful examples of this structure are found in the 

 coal seams of Ayrshire. 



In all cases of prismatic jointing thus produced, the columns 

 diverge perpendicularly from the surface of the igneous mass which 

 caused the alteration. When the latter is vertical, the columns are 

 horizontal ; when it undulates, the columns follow its curvation. It 

 is most probable that this structure is developed as the result of 

 expansion of the heated rocks. That such structure can develop 

 under pressure due to expansion is shown by an experiment in 

 which prismatic structure is formed in a box of powdered starch, 

 stored for some time in a moist region. The swelling of the starch 

 exerts a pressure in all directions against the sides of the enclosing 

 box. and after a time a series of prismatic columns is developed 

 which radiate from the center outward, being at right angles in 

 most cases to the enclosing walls. Prismatic structure produced by 

 swelling seems also to have occurred in nature, as is shown in the 

 gypsum beds of the Paris Basin (probably originally deposited in 

 part at least as anhydrite), where, as observed by Jukes, some beds 

 are divided from top to bottom by vertical hexagonal prisms. If 

 this structure is due to swelling on hydration, as in the case of the 



