12 R. A. F. PENROSE, JR. 



the coba is the great series of interbedded sands, clays, and gravels 

 which underlie the broad expanse of the pampa. 



A general section of the nitrate deposits in Tarapaca, therefore, 

 would show the following succession of formations: 



"Chuca" (loose, wind-blown material, dust, sand, gravel, etc.) . . . .o-several feet 



"Costra" (capping of the nitrate beds) 0-20 or even 30 or 40 feet 



"Caliche" (crude nitrate) 0-6 feet 



"Coba" (earthy floor of nitrate beds) Indefinite (perhaps a few feet) 



Stratified sands, clays, and gravels To great depths 



In a few places the nitrate occurs as a fringe around the edges of 

 the isolated knobs of rock that rise up through the pampa, and rests 

 either on the solid rock or on the detritus of the hillside, but its 

 usual occurrence is overlying the earthy materials described above. 



MATERIALS COMPOSING THE NITRATE DEPOSITS IN THE 

 TARAPACA REGION 



The nitrate occurs in the form of sodium nitrate with the formula 

 NaNOg, though very small quantities of other nitrates are some- 

 times found with it. It generally occurs as a translucent mass, 

 sometimes in a coarsely or minutely crystalline aggregate of rhom- 

 bohedra; sometimes in a stalactitic or mammillary form, or as an 

 efflorescence or incrustation. When pure, it is of a white color, 

 but is often yellow, red, brown, or purple from impurities. Fre- 

 quently it is much streaked or spotted a dirty brown color due to 

 sand or clay, and sometimes it is so mixed with earthy matter as to 

 have a chocolate-brown appearance throughout. The crude mate- 

 rial, as already stated, is known by the Chileans as caliche. Its 

 mineralogical name is soda niter or nitratine. Sometimes it is 

 called cubic niter or cubic saltpeter, because on casual inspection 

 its rhombohedral crystals seem closely to approach the form of 

 cubes. Commercially it is often known as Chile saltpeter, as dis- 

 tinguished from plain saltpeter, or niter, which is potassium nitrate. 



The nitrate deposits are never composed of perfectly pure sodium 

 nitrate, that material forming usually only from a small percentage 

 up to rarely as much as 70 per cent, of the whole mass. Crude 

 nitrate containing 25 per cent, of sodium nitrate is considered a fair 

 grade of raw material, one containing 50 per cent, is considered high, 



