CHAPTER II. 



ON THE FORMATION OF CAVITIES IN ROCKS. 



1.02.01. By Local Contraction. In the contraction caused 

 by cooling, drying, or hardening, both igneous and sedimentary 

 rocks break into more or less regular masses along division planes, 

 called joints, or diaclases. Numerous cracks and small cavities are 

 thus formed. Basaltic columns, or the prismatic masses, formed 

 by the separation, in cooling and consolidating, of the heavier basic 

 rocks, along planes normal to the cooling surface, are good illustra- 

 tions of the first. Larger manifestations of them often become 

 filled with zeolites, calcite, and other secondary minerals. Granitic 



FIG. 1. Illustration of rifting in granite at Cape Ann, Mass. 

 After R. S. Tarr. 



rocks and porphyries break up less regularly from the same cause, 

 but still exhibit prismoids and polygonal blocks and benches. (J. 

 P. Iddings' paper on "The Columnar Structure in the Igneous 

 Rocks on Orange Mountain, N. J.," Amer. Joitr. Sci., III., xxxi. 

 320, is an excellent discussion.) Large cracks have been referred 

 to this cause, which have afterward formed important receptacles 

 for ores. (See Example 11 a.) Very small microscopic cracks may 

 occasion lines of weakness and brecciation which are not readily 

 apparent. They afford a cleavage, called rifting, but are not well 

 understood. (See R. S. Tarr, " On Rifting in Cape Ann Granites," 

 Amer. Jour. Sci., April, 1891.) Joints are generally prominent in 



