140 Transactions of the Academy of Science of St. Louis 
boundary of the quadrangle, would if continued pass close to 
Iron Mountain. The fact that a fault zone passes by Iron Moun- 
tain is shown by the displacement of iron veins in the Big Cut. 
Crane (10) states that the abrupt termination of ore bodies in 
the underground workings at Iron Mountain suggests the influ- 
ence of faulting. The presence of a brecciated zone beneath the 
ore vein on Little Mountain (10) also suggests faulting although 
this may be a flow breccia or an agglomerate. 
The age of the faulting is pre-Cambrian and it is earlier 
than the iron solutions, for they followed the fault planes. Iron 
veins are cut by a basic dike at Iron Mountain (10), but it is 
possible that this dike is younger than most of the dikes of the 
region and it may be genetically related to the iron veins. Dikes 
in the Edgehill quadrangle are apparently cut by the faults (11) 
which may indicate that these dikes are older than the fault- 
ing or they may be faulted by a recurrence of movement along 
fault planes formed earlier. The fact that the major faults of 
the region trend northwest while the majority of basic dikes 
strike northeast is evidence that most of the dikes preceded the 
faulting. The few dikes which strike northwest may belong 
to a later period of intrusion. 
The intrusion of granite along the faults suggests that the 
faulting is older than some of the granites. The granite in- 
trusions which lie along fault planes do not have the northeast 
striking basic dikes intruded into them which indicates they be- 
long to the younger granites. At Cornwall and southwest of 
Coldwater, these younger granites contain basic dikes striking 
northwest which is further evidence that the intrusion of the 
dikes followed the faulting. 
Major Structural Units 
A number of tectonic units involving more than one type 
of structure and affecting rocks of different ages may be more 
appropriately discussed as distinct from the structures des- 
cribed on preceding pages. In most cases these units take the 
form of rectangular blocks of pre-Cambrian rocks which appat- 
ently have been pushed up through the later formations. 
