180 TRANSACTIONS OF THE [FEB. 15, 



sive crystalline formations to a line very near the main range. This 

 may indicate two distinct periods in the formation of these 

 mountains, the lower ridges on the north being the older, prob- 

 abl}'^ pre-Cambrian, and the great central upheaval an intrusion 

 through what was formerly the southern undulations of these 

 older ranges, perhaps at a much later period though still in very 

 remote times. 



Turning to the low country surrounding these mountains, the 

 Cerrajon peak across the valley gives no evidence of having been 

 intruded through the lowlands, but the foot hills along its base 

 indicate a secondary movement in the valley which may have 

 formed the watershed between the Rio Rancheria and the Rio 

 Cesar, and but for this slight elevation a part of the waters of 

 the Magdalena would find an outlet through the valley and 

 ■would reach the sea near Rio Hacha. 



Going inland from the mouth of the Rio Rancheria marl and 

 sand formations are first noticed, then as the road continues in- 

 land, alluvium is encountered which deepens conformably and 

 contains similar material growing coarser in texture right up to, 

 and across the divide. 



This could not have been deposited by the Rio Rancheria or 

 from the superficial drainage and debris from the surrounding 

 mountains, and can onl}' be accounted for by the presence of a 

 considerable body of running water, and this could not have 

 been anything but a branch of the Magdalena. 



Take away the alluvium at the delta of this river and the low 

 lying swamps and deposits to the east of it, and you have the 

 former conditions. The Sierra Nevada mountains would then be 

 an island lying off a deep embayment that has been filled in 

 by alluvium brought down b}^ the Magdalena river, and by 

 sand bars formed by the heavy currents setting along this coast 

 from east to west. 



We have in this part of South America, possibly four, cer- 

 tainly three, distinct successive stages of formation. The 

 Sierra Nevada de Santa Marta mountains represent the 

 very oldest developments ; the upheaved masses of the main 

 ridges of these mountains perhaps indicate a later stage, but 

 still in very remote times; the Black Andes, with their beds of 

 limestone which correspond to those along the base of the Sierra 

 Nevada mountains, represent the Cretaceous system ; and the low 

 lying valleys furnish the evidence of Tertiary and Quaternary 

 times. These are all well defined and offer an attractive field for 

 study, which may later claim the attention of others better able 

 to consider and work out the problems. 



Notes and reports on this region will be found in the writings 



