1897.] NEW YORK ACADEMY OF SCIENCES. 179 



all about me were sharp ridges of rock extending along the sum- 

 mit of the range with peaks at intervals, some well above 

 me, others that I could look down on. To the west, across the 

 black valley, was a small glacier, and beyond this the white peak 

 of the Horqueta shone and glistened in the sun. Just where I 

 was the snow had melted, but there was plenty in the crevices 

 and protected places. 



It was a grand experience and I could have remained for 

 hours, but it was getting unpleasantl}- cold, and presently the 

 fog began to creep closer. I hastily retraced my steps and after 

 a difficult scramble found my guide, and eventually returned in 

 safety to Pueblo Yiejo. 



Let me now call 3'our attention to a map of Colombia, that I 

 may make a brief summary of the things that especially at- 

 tracted my attention. (Plate XV at end of volume.) 



The Sierra Nevada de Santa Marta mountains are seen on the 

 north coast. The axis of these is almost at right angles to that 

 of the Andes. 



Crystalline rocks are of great development in the Sierra Ne- 

 vada as has been reported by other explorers in able accounts, 

 but their intrusion through a sedimentary^ deposit has, I think, 

 been previously overlooked. I noticed two series of physical 

 conditions among the Sierra Nevadas. In the higher elevations 

 there are bold, steep ridges along the great central uplift which 

 ai'e accompanied by broken masses, slabs of rock stripped from 

 the exposed parts, and ledges terminating in bare, sharp crags, in- 

 dicating plains of weakness and that the upheaval of these masses 

 was accompanied by a grinding and a splintering as if they had 

 been thrust through the overl3ang material. That there were such 

 overlying formations is clearly shown by the seam of sedi- 

 mentary rock on the Macatama. 



The southern exposures of these mountains are precipitous, 

 but on the north they undulate through successive ridges to the 

 sea. Among these undulating ridges I noted a second series of 

 physical conditions quite different from those of the higher 

 ranges. 



Massive crystalline rocks similar to those on the main ridge 

 formed the higher elevation in well defined zones projecting 

 from the main uplift, and these were but little eroded ; with them 

 were other ridges and hills made up of altered types, decom- 

 posing material and trappean forms. Among these latter forms 

 erosion has been extensive and the valleys are somewhat rounded. 

 In the lower mountains these gradually replace the more crys- 

 talline formations and compose the foothills, and at places this 

 -decomposing materia is found apparently overlying the intra- 



