Amphiiuans and Reptillis of Santa Marta 21 



in April and May, while the greater part of June, July, and August are 

 nearly without rain, the heavy rains coming in September and October. 

 The cause of the scarcity of rain on the south side is, at first glance, hard 

 to explain, but after a careful study of the topography and prevailing winds 

 had been made a plausible explanation was arrived at. The prevailing 

 winds are the northeast trades, which sweep in, moisture laden, from the 

 Caribbean. The south slopes of the Sierras are, to a great extent, sheltered 

 from these winds by the long spur ridge forming the southeast apex of the 

 trihedron. The trade winds are low-flying, not extending, as a rule, to 

 more than a thousand feet, or less, above the earth's surface, unless they 

 meet with some obstacle which deflects them upward. The northeast spur 

 ridge of the Nevada forms this obstacle, and its northern slope, covered 

 with a dense forest of massive trees, receives the first impact of the winds, 

 condenses the greater portion of the moisture carried by them, and then 

 deflects them upward and over it to the south slopes of the mountains, but 

 not until they have lost the greater part of their moisture, and little or none 

 is left to be condensed into rainfall there. Farther to the south the winds 

 sweep into the valley of the Rio Rancheria unopposed, and thence through 

 to the Rio Cesar Valley, the height of land between having no effect on 

 them, and having been thoroughly heated as they traversed the long stretch 

 of arid wastes composing the Goajira Peninsula, they are drying winds. 

 These strong northeast winds sweep through the great valley between the 

 Sierra Nevada and the eastern Andes, drive back the gentle, moisture-laden 

 land breezes from the Magdalena Valley and prevent their entrance from 

 the southwest beyond a certain point. To the north of the southwest angle 

 of the Sierra Nevada the moisture-laden winds from the Magdalena reach 

 the west slopes of the Sierras unopposed and discharge their moisture there, 

 giving an abundant rainfall during the rainy season on this side and its con- 

 tiguous lowlands. 



Vegetation of the Plains: The vegetation of the region under discus- 

 sion is diversified. Beginning with the lowlands, we have, on the north side 

 of the mountains, extending from the mouth of the Rio Piedras to a point 

 beyond DibuUa, a heavy growth of forest, consisting of large, tall trees and 

 very little tangled undergrowth, which extends almost to the sea beach. 

 This may be strictly termed a humid tropical forest, but it is not jungle-like 

 in its characteristics. Beyond Dibulla the forest gradually changes, merging 

 first into the "dry forest," such as is prevalent around Santa Marta, and 

 then to low, thorny scrub and cacti, prickly pear, etc. (PI. VII.) A small, 

 slender fan palm is abundant from Dibulla to Riohacha and as far south 

 as Barbacoas, growing most commonly in the open parts of the scrub on 

 low spots which are flooded during the short rainy period. This palm was 

 not observed in any other section. Near the mouth the banks of the Rio 

 Rancheria are clothed with mangroves, while higher up, above the influence 

 of the tide, the same class of trees and shrubs are found which are found 

 in similar situations around Santa Marta. 



Following the trail from Riohacha southward to Fonseca, the vegeta- 



