202 
REPORT ON THE PANAMA CANAL 
of exceptional weakness. Some of the smaller slides outside the Culebra 
District are limited by faults. 
An important source of weakness is the fracturing of the rocks by- 
complex sets of joints. They occur in both stratified and massive rocks. 
The friable parts of the Cucaracha formation are broken by joints into 
fragments of various sizes down to a fraction of an inch. The massive 
rocks are fractured in every direction by joints on a much larger scale. 
Earthquakes. — consideration of the earthquakes felt in the Canal 
Zone and a careful examination of the instrumental records kept near 
the Pacific end of the Canal since the end of 1908 dispel fear of serious 
damage to the Canal or its accessories by earthquakes. A number of 
pretty sharp shocks have been felt in the Zone but they have originated 
at a distance of about 120 miles from the Zone. A few fairly sharp 
shocks had their origin about 80 miles distant; and two or three, which 
were not felt, and which made a very feeble record on the delicate 
seismographs, were much nearer. At the time of the very sharp shocks 
of October, 1913, a prism of earth 60 feet high and with a base of about 
100 square feet broke off the steep slope near the top of the Cucaracha 
Slide; this is quite insignificant. There is no evidence that any of the 
slides have been started or increased by earthquakes. 
The Heavy Rainfall. — Investigations in many parts of the world have 
shown that excessive water in the ground is a great promoter of land- 
slides. The strength of the Cucaracha clays is greatly decreased by the 
presence of water, and the heavy tropical rains keep them nearly satur- 
ated the greater part of the year; for the broken and irregular surface 
of the slides and the open cracks aroxmd their borders greedily drink 
in the water that falls upon them. 
In the Culebra District the average rainfall since 1884 has been 87.68 
inches per annum, and during the time of the excavation of the Canal 
it has been 84.75 inches per annum. Moreover this rainfall is almost 
wholly concentrated in eight months of the year. The average for the 
rainy months during the period of excavation of the Canal by the 
United States, from the middle of April to the middle of December has 
been 80.01 inches. Where nearly seven feet of water fall upon the sur- 
face of the country withia eight months of the year, it is not surprising 
that there is difficulty in controlling the underground seepage. Indeed 
during these months the ground water table is practically at the surface, 
except possibly on the steeper slopes; and the movement of the under- 
ground water is so slow that even in the dry season the water table is 
not far from the surface in the level country. The New French Com- 
pany found that the water table at the two points of the East Culebra 
