t 
Differential Fa nits. — Ilohl>s. 
\ 
35 
the state or a region still farther to the northward, as is clear- 
ly shown by the glacial striae. The Pierce county diamonds 
are unlike the others described in that they are all small and 
occur in hexoctahedral instead of dodecahedral crystals. 
They could only be considered to have the same source as the 
others by supposing that all are derived from some area well 
toward the Hudson bay country in Canada. 
It is interesting here to note that in the Menominee region 
on the northeastern boundary of Wisconsin are basic intrusive 
rocks which cut shales containing considerable percentages of 
carbonaceous matter, thus reproducing the conditions which 
obtain at the South African diamond mines. Very similar 
conditions obtain in the Pigeon river region northwest of lake 
Superior. The Pierce county diamonds ma}' be easily con- 
ceived to have come from the latter area. It is not at all im- 
probable that productive diamond mines may sometime be 
located in one or the other of these areas. In the meantime 
diamonds will be found occasionally in the drift. There is 
little to be hoped for in the systematic search for stones in 
the moraine, as they are probably disseminated with consid- 
erable uniformity through it. This would seem to be shown 
by the occurrences that are known. 
DIFFERENTIAL FAULTS. 
By William H. Hobbs, Madison, Wis. 
The dislocations of the earth's strata are generally classi- 
fied, (1) according to the direction of the fault line with ref- 
erence to the strike of the beds, into strike faults, diagonal 
faults, and cross faults; and, (-1) according to the direction 
of hade with reference to dip. into vertical faults and faults 
with inclined hade. The latter type is further classified, (a ) 
according to the direction of throw of the hanging wall, into 
normal and reversed faults or overfaults; and, (b) according 
to the relation between the hade and the dip of the beds, into 
faults which hade with the dip and faults which hade against 
the dip. Margerie and Ileim in their admirable resume of 
our knowledge concerning faults,* distinguish two main types 
*Margerie et Heim, Lee dislocations de I' 6corce terrestre. Essai de 
definition et de nomenclature. Zurich, 1888. 
