Geological and Miscellaneous Notice of Tarapaca. 3 



the surface is sandy, and scattered over with numerous fragments 

 of pumice, basalt, chalcedony, carnelian, and agate. Between 

 Matilla and the mountain of Chalacollo, the soil is covered in 

 several places with calcareous tufa for the space of an acre or 

 more. Shrubs are here standing in the same position in which 

 they grew, the smallest twigs remaining, presenting the singular 

 spectacle of a once rank and luxuriant thicket converted into 

 stone. Gypsum, more or less mixed with fragments of shells 

 and marl, constitutes a large part of the surface of the pampa at 

 the north. In the extreme northern part it presents a very re- 

 markable appearance, being in flat rounded masses, slightly con- 

 cave on the upper surface, from five to fifteen inches in diameter, 

 and from one to two inches in thickness. They are compact and 

 hard, and contain a few minute fragments of basalt. The same 

 form also occurs in the beds of salt, which likewise constitute a 

 large part of the northern section of the pampa, but in much 

 larger and less regular masses ; presenting on a grand scale the 

 same appearance which is observed when evaporating saline soliw 

 tions, where pellicles form and fall to the bottom of the vessel. 

 These cakes of salt, many of them five or six feet in diameter 

 and a foot thick, contain little insoluble matter; they lie piled 

 one upon another to the depth of several feet, presenting a rough, 

 white, and glistening surface, over which the traveller may ride 

 all day without his horse's hoofs once touching the soil. Al- 

 though more abundant at the north, these unmixed beds o[ salt 

 are found in other parts of the province. 



In the western part of the pampa, in latitude 19° 50', at an 

 elevation of about three thousand five hundred feet above the 

 sea, and about two hundred feet above the adjoining plain, lime- 

 stone, containing shells, rises from a bed consisting of pebbles 

 and shells which are cemented together by salt, principally nitrate 

 of soda. Part of the shells are decomposed, while others are 

 perfect in form, and like those still found living on the rocks in 

 the inlets of the sea. The same variety of limestone occurs on 

 the opposite side of the mountains, near Molle, and is traversed, 

 as is also the feldspar-porphyry of the neighboring mountains, by 

 veins of the same salts which unite the shells and pebbles of the 

 plain. Among the sandstone hills on the opposite side of the 

 pampa, particularly in the vicinity of Pica, similar veins of an- 

 hydrous sulphate of soda occur. Many of them are a foot wide, 



