212 SIMYLTES, HOU (SIAQGIQIBIN IES 
to shear the limbs of the folds parallel to the bedding and to 
develop plications in the crowns and troughs. 
(2) Stnce the boundary between the zone of fracture and the zone of 
flowage ts at a different depth for two rocks of unequal strength, and 
Jor the same rock under different conditions of stress, there 1s a zone of 
combined fracture and flowage. In aset of heterogeneous beds 
upper weak strata may be in the zone of folding, while lower and 
stronger strata may be in the zone of fracture. Also, as pointed 
out by Willis, if strong folded strata at anticlines are competent 
to carry much of the load, as they often are if the arches are 
not too long, weaker strata below may be so relieved from 
weight as to be partly deformed by fracture. Thus between 
the horizon where all the rocks of a district fracture, notwith- 
standing the weight of the superincumbent beds and the horizon 
where the effective weight of the superincumbent beds, is so great 
that no rock fractures is a zone of combined folding and fractur- 
ing. This central zone is one of great thickness, and is of the 
first importance. ; 
A soft shale may be in the zone of folding, far above a strong 
quartzite or jaspilite, and the latter be in the zone of partial 
deformation by fracture. This difference in strength is at many 
places certainly equal to the weight of 2000 or 3000 meters of 
strata, and is probably equal to the weight of 5000 or more meters 
of strata. Thus this middle zone is probably at least 5000 meters 
thick, and it may be considerably thicker than this. 
In heterogeneous rock strata in this middle zone, irregular 
fracturing, brecciation, jointing, faulting, folding and the devel- 
opment of secondary structure may occur together in a most 
complex manner. A deeply buried, brittle formation may be 
under such stress that as a whole it folds without major fractur- 
ing, but in a minor way it may be faulted, fractured, or brec- 
ciated. The fracturing may leave no permanent openings, as 
the softer material may promptly flow and fill the openings 
between the more brittle broken layers. Such is the case in the 
jaspilite beds of the Lower Marquette series of Michigan. Such 
is also the case at many places where beds of shale or limestone 
