520 WARREN J. MEAD 



The classical experiment of Daubree/ in which he twisted a 

 narrow strip of glass and obtained systematic sets of fractures at 

 approximately 45° with the axis of torsion, is familiar to most 

 students of structural geology. The fractures thus developed in 

 the glass are evidently due to cross-bending. The writer has 

 repeated the Daubree experiment and found that one set of fractures 

 in the glass developed as tension-cross-bending cracks on one sur- 

 face and that the other set of fractures at right angles to the first 

 developed as tension-cross-bending fractures on the opposite sur- 

 face. Therefore, a brittle rock formation broken in the manner 

 illustrated by the Daubree experiment would show a conspicuous 

 set of tension cracks at 45° to the axis of warping and the other set 

 would not be apparent, as it would be developed from the under- 

 side of the deformed rock stratum. In other words, if we look for 

 a repetition of the Daubree experiment in the field we should look 

 for only one set of parallel tension cracks. 



Use of the apparatus in the study of folds. — Previous experimen- 

 tation in the reproduction of the structures of folded rocks in the 

 laboratory has, so far as the writer is aware, been by means of 

 the type of apparatus employed by Willis.^ This type of appa- 

 ratus with various modifications has been used by Hall, Lohest, 

 Favre, Daubree, Cadell, and others. In his investigation of the 

 mechanics of Appalachian structure Willis used an apparatus of the 

 piston or plunger type in which a series of wax layers of varying 

 consistency were built up to resemble in their relative competence 

 the rocks occurring in the Appalachian region. A load of shot 

 was superposed to simulate the weight of overlying sediments and 

 deformation was accomplished by forcing a piston or plunger 

 against the end of the aggregate by means of a screw. Willis 

 found that with flat-laying layers single folds were developed near 

 the plunger and that repeated folds could be developed only when 

 certain portions of the beds had an initial dip or when the first fold 

 next to the plunger piled up material to such thickness and strength 

 as to develop, as it were, an extension of the plunger which in 

 turn caused a secondary fold in front of it. 



' G. A. Daubree, Etudes Syntheiiques de Geologic Experimentale, pp. 507-15. 

 2 Bailey Willis, "Mechanics of Appalachian Structure, Thirteenth Ann. Rept. 

 U.S. Geol. Survey, Part 2 (1893), pp. 241-53. 



