194 STUDIES FOR STUDENTS 



casts of the irregular chambers and chimneys of caverns whose 

 walls remain as molds. Cave earth, bone breccia, incorporated 

 fragments of stalactites, and stalagmitic crusts may certify the 

 origin. The matrix is either of travertine or of limestone sand 

 and cave earth. 



Campbell 1 has described a cavern breccia exposed in the walls 

 of a canyon near Fort Stanton, New Mexico, and suggests that the 

 frequent repetition of the process might result in complete brec- 

 ciation of certain beds more soluble than the rocks above and 

 below. 



Cavern breccias are common in the lead and zinc mining regions 

 of the Upper Mississippi Valley. Fragments of sheets of ore mingle 

 with the dolomite debris. The matrix may be metalliferous, giving 

 rise to sprangle ores. Slight foundering of the strata above caverns 

 has produced crackle breccias whose fissures are healed with 

 Smithsonite. 2 



Pseudo-brecciation. — This term is used by Wallace 3 to designate 

 irregular mottlings due to partial dolomitization. Various other 

 causes produce irregular mottlings, but such can hardly be mis- 

 taken for brecciation structures. 



1 M. R. Campbell, "Origin of Limestone Breccias," Science, N.S., XXVII, 348. 



2 Whitney, Geology of Iowa, I, 448; A. C. Leonard, Iowa Geol Surv., VI, 11 f.; 

 S. Calvin and H. F. Bain, ibid., X, 480 f. 



3R. C. Wallace, Jour. Geol., XXI, 420-21. 



