654 T. T. QUIRKE BOUDINAGE, AX UNUSUAL PHENOMENON 



We may suppose, however, that the elastic limit of the quartzite might 

 have been passed and the pore spaces were crushed closed. In this case 

 the reduction might well liave been as high as 7 per cent. This com- 

 pression would shorten a bed three feet thick by 2 1/5 inches. It should 

 be noted that this compression is non-elastic and unrecoverable on relief 

 of pressure. A certain amount of elastic compression must accompany 

 compression so great that the pores are closed. Recovery of this would 

 be possible on relief of pressure. 



After closing of the spaces the quartzite would have an elastic com- 

 pressibility about equal to that of quartz. This is very small, being 

 about 2.7 * parts per million per atmosphere. 



Supposing the compression to have been equal in intensity to the 

 pressure produced by rock overburden 10 miles deep, the compression of 

 the quartzite would have been of the order of about one-third of an inch 

 per linear yard (0.02 per cent). Thus a quartzitic bed under great com- 

 pression might be reduced 2 1/5 inches per yard non-elastically and one- 

 third inch per yard elastically, resulting in a total shortening of 2 8/15 

 inches per yard. This shortening could be accommodated by the outside 

 beds by arching. 



By experiment and measurement, it has been found that a yardstick 

 shortened by bending 2 8/15 inches results in an arch equivalent in 

 height to one-seventh of its original length or to one-fifth of its reduced 

 length. This is an arch of the shape of the outside layer of the boudins 

 drawn in figure 5. 



Supposing the boudins to have l)een formed by lateral forces which 

 resulted in elastic and non-elastic compression of the axial parts and 

 ordinary folding of the outer layers, there would have been a linear 

 shortening of 2 8/15 inches per yard, of which 2 1/5 inches could not 

 be recovered ]3y expansion after relief of pressure. There would also be 

 a thickening of the beds equal to nearly 10 inches in the widest part of 

 the outside layers, arching upward and downward. 



After relief of j^ressure the arches might straighten in part, but the 

 central region has little recovery possible. If the arches lengthen again 

 under vertical pressure or decrease of lateral support, the central part 

 must crack open. These cracks would be greatest at the center, where 

 arching Jias ])een zero and where non-elastic compression has been a 

 maximum. The cracks would have a maximum width of 2 1/5 inches 



* Erskine D. Williamson : Changes of the physical properties of materials AA'ith pres- 

 sure. Geophysical Laboratory, Carnegie Institute of Washington, Paper no. 446, 1922, 

 p. 506. 



