REPORT OF BOARD OF CONSULTING ENGINEERS, PANAMA CANAL.


gravel covering of the rock bed is about 50 feet in depth. At San Pablo the bed of gravel, sand, etc., is about 90 feet in thickness; at Buena Vista it is over 139 feet below sea level to rock, and a little farther downstream, 142 feet; at Bohio the rock is 168 feet below tide level, and the drills penetrated wood at various depths to 150 feet. At Gatun, the depth to what in this report is sometimes classed as rock-an indurated sandy clay-is 258 feet in the deepest place, and at another, on the same section, the depth of the sand, clay, gravel, etc., is about 240 feet. Here also buried wood was brought up by the drills.
 It seems, therefore, to be certain that what may be called the geological valley of the Chagres-that is to say, the rock bottom of that stream-is represented by a deep groove or channel, now entirely or partly filled by the products of erosion and drift. If there has been a regional subsidence of the Isthmus, which the geologists suggest as possible, it may be that the ancient Chagres discharged into the sea through an ancient valley, which, with the land adjacent thereto, was some 300 feet higher in relation to the ocean than the present valley. The rock penetrated at Bohio and above, also that showing in the river banks and outcropping in neighboring hillsides, is all volcanic and much denser than the so-called rock at Gatun.
 The Obispo flows over a rocky bed in a part of its lower course and at one point there is a natural cascade of a few meters over a rock shelf. The general surface on the upper course of the Obispo is more nearly level, with hills rising in its drainage basin and on its margins to the height of from 100 to 1,000 feet above the general surface. The general aspect of the central isthmus is one of great irregularity-the hills are numerous and have very steep slopes, while the valleys are narrow. Culebra is one of these hills, its summit being some 700 feet above the sea. The geologists suggest that the drainage area of the upper Obispo was once a lake of considerable size, for within this area are found a few sedimentary rocks containing fossils, also calcareous and carboniferous deposits, but the greater part of the material is of volcanic origin, the central masses of the hills containing hard volcanic rock and dikes of basalt.
 Between, above, and below these hard-rock masses are softer rock and dark, indurated clay, while the upper covering of the supericies is composed of the same volcanic material, but much changed by exposure to the weather, and where cut through, as it is by the canal excavation all the way for seven miles through the dividing ridge, the covering is seen as a red clay, occasionally containing bowlders, having a varying thickness to 30 feet and at one place to more than 40 feet, but generally its thickness is but 10 to 20 feet. It is in this top layer of reddish clay that all the larger slides have taken place.
 Toward the Pacific the slope is, for half the distance to the bay, much more rapid than in the Chagres Valley, but the physical characteristics are similar. The Rio Grande Valley, through nearly half its length, is a tidal estuary filled and emptied twice daily by the tides. The same or a similar rock to that showing in the upper Chagres Valley is the prevailing rock in the Rio Grande region, with but a few feet of earth covering. At Pedro Miguel and Miraflores the rock is near the surface. Near Panama are two isolated hills of considerable height showing volcanic rock outcrops of a very much denser character than any other on the Isthmus.
 The facts being as stated, it follows that the streams draining the isthmian region have a much more permanent regimen in their upper courses than nearer the sea. The rock near the surface a few miles from the oceans is conveniently situated for foundations for locks, dams, etc., and is sufficiently dense to make good concrete material, while sand suitable for use in masonry structures is found in great abundance on the beaches of Panama Bay, and probably that found in some of the gravel beds in the Charges will also be suitable for the same purposes.
 The hill and mountain slopes are covered with a tropical jungle, but there is no good timber for construction purposes found on the Isthmus near the railway.

 CLIMATE.
 The climate of any locality is determined by certain influences, the principal of which are latitude, altitude, proximity of oceans, high mountain ranges, humidity, and rainfall.
 A detailed description of the climate of the.American Isthmus is quite unnecessary, for it is well known, but its adaptability as a residence for human beings employed in manual labor or


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