JOINTS 791 



siliceous lutytes. It consists of a "delicate tracery of feathery lines 

 diverging from a roughly outlined axis, which traverses the face of 

 the joint hlock in a plane parallel with the stratification. When the 

 axis of the feather fracture departs from this plane it becomes sinu- 

 ous." (Woodworth-26.) 



Dendritic markings. These are formed on the joint planes of 

 fine-grained rocks, and are due to the arborescent deposit of earthy 

 oxide of manganese or of iron. They may be compared to the 

 plumose frost traceries on window panes. In the so-called land- 

 scape marble, this deposit has penetrated the entire rock, and is seen 

 on all polished sections. Dendrites of iron pyrites and other min- 

 erals are also known. Not infrequently they are mistaken for 

 vegetable impressions. 



^ Widening of joints. 



Joints of tectonic origin may be widened by separation of the 

 blocks, by solution or erosion of their sides, and in other ways. The 

 Olean conglomerate on the hills of southwestern New York fur- 

 nishes a good illustration of widening of joints by separation of the 

 blocks. Here huge masses 40 feet in height and of similar basal 

 dimensions have been formed in the coarse conglomerate, and the 

 blocks have in many cases slid far enough apart to open passage- 

 ways between them. These street-like passageways between the 

 blocks have given the region the name of Rock City. The gliding of 

 the masses is favored by the soft, clayey strata underlying and by 

 the constant erosion which is going on on the hillsides. 



Joints in limestones are commonly widened by solution of the 

 wall rock, which when continued long enough will produce cavern- 

 like passages and eventually caves. The peculiar character of the 

 Karst regions of the world is accounted for by such solution. 

 (See ante, p. 133.) Not infrequently partly widened fissures are 

 filled with clay, sand, gravel or other substances from above, con- 

 stituting a clastic "dike." Sandstone dikes are especially common, 

 though not always originating in solution fissures (see beyond). 

 Fissures thus filled are common in most modern limestone masses, 

 covered by drift deposits. A remarkable example of a Devonic fis- 

 sure in Niagaran limestone, filled with fossiliferous Upper Devonic 

 shales, has been described by Weller from Illinois (25). 



Widening of joint fissures by erosion is of common occurrence. 

 The most active agent is often the wind, which cuts away at the 

 sides of the prism produced by the joints, and narrows it until only 

 a pillar, isolated from its neighbor, remains behind. All stages of 



