428 



TERENCE T. QUIRKE 



side, became nearly horizontal near the center, and increased to 

 nearly 60° as it approached the upper surface (Fig. 2). To prove 

 that this is the usual t)^e of break and not fortuitous, two more 

 pieces of soap were flexed and broken with similar results. Two 

 narrow pieces of soap broke with nearly vertical shear planes near 



Fig. 2. — (o) A sheet of soap ruptured by combined compression and bending 

 under rotational stress. The fault plane is high-angled near the surface and low- 

 angled below the center of the member. These pieces are outlined in Figure 3. {h) A 

 sheet of paraffin ruptured by combined compression and bending under -rotational 

 stress. The fault plane is almost horizontal for most of the length, changing sharply 

 at a high angle to the surface. At one end the member is split down the center along 

 the plane of maximum shear without actual breaking out of the piece; see also Figure 5, 

 (a) and {b). 



the edges (Figs. 2 and 3), but a plate wider than it is long ruptured 

 without vertical shear planes at a low angle (Fig. 4). It seems 

 that nearly vertical fault planes may be disregarded, occurring 

 merely because the sheets are narrow and have lateral rehef. In 

 the case of earth deformations in general, the width of the sheet 

 or strata-plate is probably comparable to the length of the defor- 



