All the same, the Gatun dam has two extremely responsible and heavy duties to perform. It has to withstand the horizontal thrust of a head of 85 feet of water so as not to be carried bodily down the Chagres bed into the Atlantic. And it has to block up the valley so effectually that the water of the lake shall not percolate through at any point. There is every reason to believe that, in spite of all alarums and excursions during its construction, it will fulfil both these requirements. Its composition and construction may be briefly described. Two bulwarks of big rocky fragments were built up on either outer line or "toe" of the structure. This rough material was obtained from the lock site, or Mindi, or the Culebra Cut twenty-six miles away. The area between these piles is filled with silt, and water pumped into it by hydraulic dredges from the Chagres valley. The surplus water is carried off through pipes. The sodden silt remains and is packed down and consolidated by atmospheric pressure. Such a "hydraulic fill" is impervious to water, the thrust or "head" of which is very quickly lost in the minute interstices or pores of the material. It will be seen how such a structure differs from a dam of concrete or stone masonry. It is porous, while at the same time impervious to water. The future traveller through the Panama Canal will probably never guess the immensity of the labour that has gone to the building of the Gatun dam. Already, indeed, it looks so much like a part of the natural landscape that it might well escape special observation altogether. Yet nothing less than 21,145,931 cubic yards of material were laid down--enough to make a wall of earth three feet high and three feet thick reaching nearly halfway round the world. The spillway itself contains 225,485 cubic yards of concrete.