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PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



[Nov. 21, 



does not exceed 600 feet above the level of the sea. An occasional 

 narrow strip of sea-beach is seen bordering this range, and is com- 

 posed exclusively of the debris of the mountains themselves, which, 

 being in many places nearly perpendicular, expose to the spectator, 

 passing along in the coast-steamers, a fine section of the shales, 

 claystones, and imbedded porphyries which here represent the Upper 

 Oolitic series, and are occasionally seen disturbed and altered by the 

 intrusion of dioritic rocks of a later age. 



At Arica, however, this range of mountains suddenly recedes from 

 the coast, leaving an intermediate space, about 30 miles broad and 

 extending as far northward as examined, occupied by gently sloping 

 plains, evidently ancient sea-beaches, which rise to the height of 

 about two thousand feet above the sea-level. These plains, being 

 for the most part entirely destitute of water (as no rain falls in these 

 regions), are consequently entirely barren, and present to the eye of 

 the traveller a most desolate and arid appearance. "When, however, 

 as in the valleys of Tacna, Sama, and Azapa, a scanty supply of 

 water does occur, the soil, noted for its surprising fertility, produces 

 a most luxuriant vegetation of a semi-tropical character. 



These plains are composed of sand, earth, and gravel, with abun- 

 dant fragments, more or less rounded, of the porphyritic, dioritic, 

 and volcanic rocks forming the coast-range of mountains which 

 bound them to the eastward. Even after a most careful examination, 

 no single fragment or boulder of any extraneous rock was met with ; 

 so that no drift- action appears to have assisted in the formation of 

 these beaches, which appear due solely to the action of the waves 

 beating against the former rugged line of coast. 



In the Sections No. 1 and 2, PI. II., which cut through this district, 

 it will be observed that a volcanic formation, apparently contempora- 

 neous, is situated in the midst of these plains, which does not bear 

 the appearance of having been injected into the diluvial beds forming 

 them, but rather to have flowed over them, or more probably to have 

 been deposited on the top of them whilst still under water, in the 

 form of a tuff or volcanic ash, and subsequently to have again been 

 covered up by similar diluvial matter ; a more detailed examination, 

 however, is necessary to settle the question. I may mention that 

 Dr. Vance of Tacna informed me that near the railway at that place 

 the ground in a cutting was found to be burnt and altered, as if by 

 igneous action ; this appears to me as more probably due to still 

 later volcanic activity. 



The rocks of this volcanic formation are all trachytic, and fre- 

 quently present a most striking similarity to the domite of Auvergne, 

 being, like that, composed of quartz, black or brown hexagonal mica, 

 and a weathered-looking felspar, and form some four or six beds, 

 superposed one on another, and of an average thickness of about 10 

 feet each ; these are either a white trachytic tuff, like domite, with 

 abundant imbedded fragments of pumice, or a compact trachyte of a 

 reddish or white colour and similar composition. 



These trachytic tuffs form an excellent building-material, from the 

 ease with which they are worked and shaped, and arc very exten- 



