46 proceedings: geological society 



surfaces are mantled by a heavy deposit of bowlder-gravels. The bowl- 

 ders range from one to five feet in diameter and extend at least fifty 

 miles from the base of the range. Even at that distance some of the 

 bowlders are three feet in diameter. These deposits are interpreted 

 as portions of alluvial fans spread out upon the plateaus about the mar- 

 gin of the San Juan dome, following the redoming of that area and the 

 associated deformation of the peneplain. Subsequently to the laying 

 down of these gravels there were at least two distinct stages in the dis- 

 section of the plateau prior to glaciation. The earlier of these has been 

 called the Bowlder-mesa stage. During that time broad areas were 

 reduced to the local base level of erosion and strewn with vast quantities 

 of gravels. The later stage has been called the Oxford stage. During 

 this period the areas of softer rocks were reduced to late maturity or 

 early old age in erosion topography. In the Bowlder-mesa and Oxford 

 stages the mountain canyons were deepened and widened and remnants 

 of these ancient valleys appear to-day as benches in the mountain can- 

 yons. Succeeding these erosion intervals, associated with which there 

 were evidently periods of mountain growth in the San Juan region, was 

 the glacial period. During the glacial or inter-glacial epochs moun- 

 tain growth probably continued and there is reason to believe that some 

 movement occurred after the disappearance of the last glacier. 



Geological features bearing on the construction of the Panama Canal. 

 C. W. Hayes. 



This paper reported the results of a brief visit to the Canal Zone made 

 for the purpose of determining the cause of and possible remedy for slides 

 occurring in the side walls of the canal, which were giving much trouble 

 to the engineers in charge. 



In general the side slope practicable in any excavation depends on 

 (1) depth, (2) character of materials, and (3) geological structure. 

 Examination of the cut indicates that serious difficulty through failure 

 of slopes will be experienced only in the section between Empire and 

 Pedro Miguel. Small slides will occur outside of this section, as at 

 La Pita, but they are due to exceptional conditions and will be relatively 

 unimportant. 



Within the section defined above are four classes of materials: (1) 

 Sedimentary rocks . This class includes (a) volcanic agglomerates ejected 

 from volcanoes in the form of angular fragments varying in size from the 

 finest dust to several inches in diameter; deposited in water but very 

 imperfectly bedded; generally bluish green; moderately hard when first 

 exposed but crumbling rapidly in contact with the air. (b) Fine blue, 

 red or black clays, composed largely of volcanic dust which has been 

 transported and deposited by water and is well bedded; like the agglom- 

 erate, moderately hard when first exposed, but crumbling even more 

 rapidly in contact with the air. (c) Lignite coal or black carbona- 

 ceous clay; beds one to three feet thick interstratified with (b). These 

 beds (a), (b) and (c) with some unimportant sandstones and limestones 

 constitute the Culebra formation of Tertiary age. (2) Intrusive igne- 

 ous rocks: These are chiefly large masses of dark andesitic rock which 



