THE EARTHQUAKE AS A NATURAL PHENOMENON. 7 
CRACKS. 
All through the area of high intensity cracks were made, and these 
were specially numerous near the fault trace. The cracks were 
also more numerous in soft alluvium than in hard ground, but the 
number which deeply penetrated the bed rock was large. Perhaps 
this feature is better expressed by saying that the bed rock was gen- 
erally and profoundly shattered, but without important dislocation 
except on the old fault plane. The wide prevalence of shattering is 
shown by the derangement of the underground circulation of water. 
In every farming district within the main earthquake belt persons 
familiar with the springs noted changes in the flow of water, rang- 
ing from moderate diminution or increase to complete stoppage or 
to the breaking out of new springs. In some places the derange- 
ments were temporary only, but more commonly a permanent change 
was reported. 
At the surface the cracks had great variety of expression. Some 
were barely perceptible as partings; others gaped so widely that one 
might look down them several yards. Some were mere pullings 
apart; others showed small differential movements of the nature of 
faulting. Some were solitary; others, especially those exhibiting 
faulting, were in groups. Some straggled and branched irregularly; 
others were nearly straight for hundreds of feet. Theoretically, some 
cracks were primary as regards the earthquake and others secondary; 
that is to say, some were caused directly by the preexistent stresses 
which produced the main fault and others were caused by the waves 
constituting the earthquake. 
PI. IV shows two types of secondary cracks. In .1 the cracks are 
crooked and without faulting. They traverse tidal mud near the 
head of Bolinas Lagoon and are near the main branch of the fault 
trace, which follows the base of the bluff seen at the right. The 
cracks seen in B are in Bolinas, within half a mile of the supposed 
position of the fault trace. The greater part of the village lies in a 
narrow valley dividing a plateau. The valley floor is of alluvium, 
the surface of which curves upward at the sides. As a result of the 
earthquake the alluvium settled somewhat toward the middle of the 
valley, thus forming along the bases of the hills a system of cracks 
associated with faulting. 
DISLOCATIONS OF SURFACE MATERIAL. 
The district most strongly affected by the earthquake is one in 
which landslips are normally of frequent occurrence. On many hill 
slopes Avere masses of earth or rock, the descent of which was sure to 
take place whenever conditions became favorable. The shaking of 
