BEHAVIOR OF INDIVIDUAL STRUCTURES. 101 
A great many marble slabs were thrown down by the concussion 
due to the dynamiting of dangerous walls at Seventh and Market 
streets, less than half a block away. The damage to marble finish 
from this cause was much greater than that due to the earthquake. 
Although the slabs brought down by the dynamite may have been 
loosened by the earthquake, they could have been taken down and 
reset if the dynamite had not caused their total loss. In one sense 
much of the loss was irreparable, for adjacent slabs were beautifully 
matched by being sawed from the same block and then so set in 
groups of tw T o or four that the veining was symmetrical about the 
joints. Where one slab of a group w T as destroyed the only way to 
restore the finish to its original beauty would be to renew the whole 
group. In addition to injuring the marble finish, the dynamite did 
much damage by blowing windows, transoms, and doors from their 
frames, and by blovdng panels out of doors and glass out of wundows. 
With the kind of woodwork used in this building and with plate 
glass in all the windows, this item of damage w T as very heavy. 
Debris from the demolished buildings, also, was thrown across the 
street and came down through one or two sk} T lights in the post-office 
building. It was only owing to good luck that some of the employees 
were not killed or seriously injured. As it was, the interiors of one or 
two expensively finished rooms were almost wrecked. All the items 
of damage which Mr. Roberts ascribed to the dynamite were of the 
same general type as those due to the concussion in mortar batteries, 
etc. The chief engineer of the building stated that he and some of his 
subordinates, although down in the basement, were thrown to the 
floor by the force of one of the dynamite explosions, and that the 
electric lights were extinguished for several seconds, although the 
engines kept running. Probably the concussion lifted the brushes 
from the commutators of the generators. It seems probable, from 
what evidence I was able to gather, that at first the dynamite was 
used in rather large quantities without tamping; but later it was 
used in a more tentative way and tamped with sand bags. By this 
policy those in charge of the later blasting operations finally got 
the work down to a system whereby a wall was brought down within 
a distance of 20 feet from the base on either side without any dan- 
gerous hurling of debris beyond that distance. So far as I was 
able to learn, after the work had reached this stage no one noticed 
any damage to neighboring structures, and, in fact, very few people 
were aware that blasting was in progress. 
Some of the mosaic ceilings in the post-office building were planted 
on a terra-cotta base, others on a furring of metal lathing and plas- 
ter. The former were seriously damaged by the earthquake, but 
the latter remained intact. As previously stated, the interior walls 
7171— Bull. 324— 07 S 
