1909] on the Americans and the Panama Canal. 695 



mental dams in order to thoroughly ascertain the reliability of the 

 available materials as well as of the soundness of the methods 

 employed in the construction of the G-atun dam. 



The site of the proposed dam at Gatun has, I think, been 

 minutely surveyed, and complete information regarding conditions 

 underground has been obtained by numerous test-pit borings. The 

 surface soil is naturally weathered and more or less disintegrated by 

 vegetation, but soHd ground is found by stripping the area under the 

 future base of the dam to a depth of not more than 1| to 3 ft. 



Two deep test pits have been dug. The strata encountered were 

 similar to those found in drilling and excavation both in the spillway 

 and lock site. They were badly seamed. Quantities of water leaked 

 through the seams. It is believed that the bulk of the water is held 

 in the disintegrated portions of these strata and finds a passage from 

 one plane to another along the joints. 



Forty to fifty feet below sea-level a layer has been found of ryolite 

 apparently forming a firm and solid stratum, and it is believed that, 

 by letting down a curtain wall as far as this layer, ample protection 

 will be obtained against any appreciable entrance of water from the 

 lake through the seams and cracks. 



The second test-pit in the centre of the dam 80 feet below sea- 

 level (this means 90 feet lower than the surface of the ground) showed 

 from the surface the following successive strata : brown sandy clay, 

 blue and yellow clay, blue sandy clay, a stratum of blue clay with 

 decayed vegetation, and down at 60 feet below the surface a stratum 

 of blue clay with shells. Comparatively little water was encountered. 

 From other borings it would appear that the entire section of the 

 Gatun Gorge upon which the dam will be built is composed of mate- 

 rials similar to those exposed in this pit. 



Cross sections are given to illustrate this paper in order to show 

 the geological formation under the centre of the Gatun Dam axis. 

 One drawing shows the formation 300 feet south of the dam axis ; 

 the other diagram represents the boring cross-section 400 feet north 

 of the dam axis. 



The vertical continuous double lines show the borings made since 

 December 1907. The vertical dotted double Unes mark the borings 

 previous to December 1907. 



Drilling has been employed in order to bring geological samples 

 to the surface, and this method, Avith a continuous drive sample from 

 the surface for purposes of comparison, has given the best infor- 

 mation regarding the materials underground. Ryolite is perhaps thf 

 most impervious rock found at Gatun, and does not readily yield to 

 disintegration. As a rule all the conglomerates found at Gatun 

 weather badly and soon crumble to gravel when excavated. They 

 are solid enough when protected from the atmosphere. 



Much has been said of " artesian water," or water under pressure, 

 occurring in the Gatun Valley. A great number of wash-drill holes 



YOL. XIX. (No. 103) 2 z 



