494 



APPENDIX 



the medanos crossed the plains obliquely in the same north- easterly 

 direction for about three miles to the foot of the mountains, ascend- 

 ing the lower slopes about 500 feet, and here the sandy area termi- 

 nated. In their traverse of the plains this column of medanos 

 crossed two hill ranges that rose 100 and 150 feet above the plains. 



By careful measurements on three medanos I ascertained that in 

 five days they had advanced about a foot, the prevailing winds 

 being light and from the south-west. From time to time slides take 

 place down the steep face of the concavity, the sand caking a little 

 on the surface and forming layers, a half to an inch thick, that 

 slide to the bottom. Impelled by fresh winds, the medanos may 

 move yards daily, and when driven by violent winds, as we learn 

 from Dr. von Tschudi, the medanos pass rapidly over the plains. 

 Strewn over the ground all over the medano region is a much coarser 

 sand that could be moved only by strong winds. It is arranged in 

 wavelets about two feet apart and one to three inches high, and 

 remains at rest when the lighter medano sand is moving briskly 

 along. 



Measurements of the Sand -grains in the Region of Medanos (moving 

 Sand-dunes) in the Ancon Coast-district of Peru. 





Finest 

 Material aver- 

 aging "2 mm. 



in size 



Medium 

 Material aver- 

 aging - 3 mm. 



Coarse 

 Material aver- 

 aging 5 mm. 



Extra-coarse 

 Material aver- 

 aging 1-2 mm. 



Sand blown through the 

 air a foot above the ground. 



95% 



5% 







Sand blown along the 

 surface of the ground. 



80% 



20% 







Sand of a typical meda- 

 no four miles from the 

 starting-place above the 

 beach. 



55% 



44-5% 



0-5% 





Sand of a typical meda- 

 no one mile from the 

 starting-place above the 

 beach. 



41% 



58% 



i-o% 





Drift sand blown up the 

 hill-slopes 30 feet above 

 the beach just mentioned. 



11% 



69% 



20% 





Sand from the wavelets 

 spread over the medano 

 plains. 









100 % 



Note. — The prevailing winds were light with an average force of three. 



The sand is derived from the disintegration of andesitic rocks. It 

 is composed in their order of frequency of grains of felspar, magnetite, 

 semi-vitreous volcanic rocks, pyroxene, quartz, brown mica, horn- 



