FISSURES, OR FRACTURES. 



227 



boidal; in shale, long, parallel, straight ; in limestone, large, regular, 

 cubic ; in basalt, regular, jointed, columnar ; in granite, large, irregularly 



Fig. 193.— Kegular Jointing of Limestone. 



cubic, or irregularly columnar. On this account a perpendicular rocky 

 cliff usually presents the appearance of huge, irregular masonry, with- 

 out cement. 



The cause of joints 

 is probably the shrink- 

 age of the rock in the 

 act of consolidation from 

 sediments (lithification), 

 as in stratified rocks, or 

 in cooling from a pre- 

 vious condition of high 

 temperature, as in the 

 igneous and metamor- 



pllic rocks.* Fig. 194.-Granitic Columns. 



Fissures, or Fractures. 



These must not be confounded with joints. Joints are cracks in 

 the individual strata or beds; fissures are fractures in the eartlCs 

 crust, passing through many strata, and even sometimes through many 

 formations. The former are produced by shrinkage and perhaps other 

 causes ; the latter by movements of the earth's crust. Fissures, there- 

 fore, are often fifty or more miles in length, thirty to fifty feet in width, 

 and pass downward to unknown but certainly very great depths. They 

 often break through the crust into the sub-crust liquid. 



* Daubree thinks that joints are due to crust-movements, especially by torsion ; and 

 therefore that there are all gradations between joints and great fissures (Geologie Syn- 

 thetique). 



