134 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
to its end ; as faulting continues, the length of the block may increase, 
and its end will thus vary in position. The faults may be simple or 
complex ; the faulted front of a block may be clean cut, stepped, or shat- 
tered; the fault line along the mountain base may be essentially indifferent 
to the structure of the block, for the fault may be of deep-seated origin and 
not necessarily guided by the pre-existent foliation, stratification, folding, 
or faulting that is seen in the upper part of the block. The fault surface 
may be nearly a plane or a conspicuously curved surface, but from all 
that is known of faults it cannot possess sharp or exaggerated irregular- 
ities such asare seen in the septa of an ammonite. The uplift and tilting 
may vary widely in the different examples of a single district. Appro- 
Ficure 2. 
Diagram of a lifted block; youthful stage. 
priate to all these variable elements, the present form of a faulted block 
may exhibit little or much modification by erosion ; little modification 
being consistent with rapid and recent faulting of a resistant block in 
very arid climate; much modification being consistent with slow and 
ancient faulting of a weak block in a climate of sufficient rainfall to pro- 
duce active erosion. In a block whose length has been increasing during 
a long time of increasing displacement, it would be reasonable to expect 
a mature dissection near the middle of the block to give way to young 
dissection near the ends of the block ; for the middle part will have been 
long exposed to erosion while the ends will have been but lately uplifted. 
The prefaulting form of the block surface will usnally be longest pre- 
served near the base of its exposed back slope, cp, and the form due 
immediately to faulting will be best seen near the base of the front, cB. 
