144 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
will be first considered, and after this the form of the spurs between the 
valleys. 
It follows from the scheme graphically represented in Figure 1, that a 
very rapid and modern faulting of a very resistant rock mass in a very 
conservative climate would produce a mountain block having a notably 
smooth fault-face or escarpment along its front. On the other hand, the 
gradual and long-continued faulting of a weak rock mass in a destructive 
climate would produce a mountain block having well-developed ravines 
and canyons whose erosion had been accomplished during the progress 
of the faulting. The essential characteristic of such ravines would be a 
Y-like cross-section even down to the ravine mouths ; and as long as the 
uplifting of the mountain block actively continues, the streams that are 
dissecting it cannot widen their valley floors. Indeed, many of the 
smaller streams might be unable under such conditions to attain a graded 
slope even in weak rocks, and their channels would be marked by rapids 
near the base line of the mountain, where the V-ravine would suddenly 
open upon an alluvial fan, sloping gently forward to the waste-covered 
piedmont plain. It is only after faulting has ceased that the streams 
can advance in an interrupted progress toward mature development and 
widen their valley floors toward the mountain front ; and only after fault- 
ing has long ceased can the valley floors be so far developed as to leave 
nothing but residual skeletons of the original mountain block between 
them. Variations in rate of faulting and in resistance of rock masses 
would produce many corresponding variations in ravine forms, many of 
which may be easily deduced, but none of which demand immediate 
consideration. 
The points that need special emphasis in this connection are that the 
characteristic form of ravines and canyons, carved in a faulted mountain 
block during the progress of a long-continued and still active faulting, 
can be reasonably determined by deduction ; that these forms are well 
specialized ; and that their most notable peculiarity is the persistence of 
a V-section down to the mountain base, where the steep-walled ravine 
or canyon suddenly opens upon a gravel fan that slopes forward to the 
wide piedmont plain. For example, the canyon of Plate 5, B, opens 
on the face of Plate 6, B. It goes without saying that this peculiarity 
of canyon form is impossible in a residual mountain, carved by the ex- 
tensive erosion of a once much larger mass, unless the most special con- 
ditions conspire to produce it. Such conspiracy is found, as has been 
said, in the stratified Appalachians, where the belts of resistant sand- 
stone, interstratified with much weaker shales and limestones, now stand 
