150 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
ing of the upper front edge of the block now remains, for the flaring 
walls of the ravines now meet in a sharp ridge crest that rises backward 
from the vertex of a triangular facet on the block front, toward the 
top of the block. In the third stage, Figure 8, the block is raised 
Figure 8. 
Diagrams showing spurs and deep ravines in the front of a tilted block, uplifted so high 
that nothing of the upper surface is here seen. 
still higher, and the ravines have become still longer and deeper ; at 
this stage the mountain crest might become serrate, and its back slope 
would be well dissected. The long sharp-crested ridges between the 
larger front ravines are still terminated by triangular facets, very 
systematic in form and position, with their bases aligned along the 
mountain front. The spur sides and the facets themselves will have 
