ISOSEISMALS: DISTRIBUTION OF APPARENT INTENSITY, 237 
of the destructive effects being obliterated, along with the structures in which they were 
developed. Enough remained, however. Foundation walls and sidewalk pavements 
were broken and flexed; sharp little anticlines were produced in the street by the arch- 
ing of block paving, as on Russ Street between Folsom and Howard Streets (plate 88c) ; 
granite curbing was broken and thrust up into an inverted V, as on Moss Street, between 
Folsom and Howard Streets (plate 88p) ; there were fissuring and slumping in the block 
pavement, as along Columbia Street between Folsom and Harrison Streets (plate 89a), 
and sharp flexures of the paved streets and car tracks, as on Sixth Street just south of 
Howard Street. These effects point simply and clearly to the great magnitude of the 
intensity thruout the greater part of this old swampy district. 
Attention has already been directed to the slumping or flow movement to the east 
along the long axis of the area. 
The heavily ballasted car-tracks on Bryant Street, at the crossing with Fourth Street, 
were sharply flexed laterally, tho bounded by block paving. (Plate 89n.) This was at 
the eastern end of the district where the marsh formerly bent to the south around the 
flanks of Rincon Hill, a mass of firm sandstone rising from the floor of Mission Valley. 
No similar sharp flexures were encountered along east-west streets in the western or cen- 
tral portion of the district, tho lateral displacement and flat, sinuous curvings of the street 
lines were common enough; notably on Harrison Street between Fifth and Sixth Streets, 
and on Folsom Street between Fourth and Seventh Streets. Both these streets cut across 
the direction of the flow movement at a small angle. These phenomena are easy to 
understand if, as seems certain, Rincon Hill served as a solid buttress against which the 
flow to the east was arrested, causing sharp crumpling of the surface near the buttress, 
with less disturbance farther away. This was combined with a slight tendency to flow 
southward in the southeastern part of the district. 
The shaking caused the materials used in filling to settle together and occupy less space, 
so that the surface over the whole district was lowered by amounts varying from a few 
inches to 3 feet or more. This is clearly seen in the change of street levels along the 
margin of the solid ground, where the car rails are bent downward in little monoclines. 
Occasionally a structure with a relatively good foundation remains at its former level, 
with the whole neighborhood deprest about it. Such a case is exemplified on Sixth Street, 
a little south of Howard Street, near the margin of the area. (Plate 89c.) The flow 
movement is thought to be due simply to the action of gravity, the loose, water-soaked 
material being compacted into less volume by the shaking. Besides this sinking of the 
district, and its flow movement, mention has been made of the deformation of its surface 
into irregular waves, trending approximately east and west parallel with the length 
of the district. Along the streets running approximately north and south, at right angles 
to the elongation of the area, car rails were bent abruptly to the side, or raised in arches, 
and sharp anticlines were formed in the block pavements. Large square concrete slabs, 
used for sidewalk paving, were thrust one over the other; and in one or two cases a slab 
entirely covered an adjoining one. These phenomena indicate shortening by compres- 
sion in the north-south direction. On the other hand, however, a stretching of the sur- 
face is shown by fissures in the paving; by places where wedge-like blocks were deprest 
below the general level; and by the rails of car tracks which were pulled apart in amounts 
varying from 8 to 12 inches. Owing to the relatively great and very variable structural 
strength of paved streets and heavily ballasted car tracks, these phenomena are not 
developed regularly nor frequently enough to afford a satisfactory test of the hypothesis 
that they are directly associated with the wave forms into which the surface of this dis- 
trict was thrown. Besides, owing perhaps to the varying rigidity of the materials which 
make up the surface of the streets and building plots, the wave forms themselves, tho 
generally prevalent, are not persistent in their extension. The compression and disten- 
