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PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



[Nov. 21, 



A little to the north of Cobija, where, from the configuration of 

 the coast, the line of dioritic eruptions previously described runs 

 into the sea, we find these forming small pointed or rugged rocks 

 jutting up from the sea, as has been noticed and figured by Von 

 Bibra* and D'Orbigny f. 



M. d'Orbigny even supposes these form part of the rock " la plus 

 ancienne de l'ensemble," and as pre-existent to the upheaval of the 

 Cordilleras. A careful examination, however, proved to me that they 

 are only, as before stated, a part of the general line of diorites, and 

 consequently younger than the Oolitic series which at these very 

 places they penetrate into and alter as before described. 



The diorites on Section No. 2, at Chuntacollo and Guanuni, are 

 classed and coloured by M. d'Orbigny as granite, thus confounding 

 under one head rocks which, beyond their common igneous origin, 

 are neither in external appearance, mineral character, chemical com- 

 position, nor geological age in any way allied. As far as my re- 

 searches have gone, I have not met with any granite in South 

 America which can be proved to be of later geological age than the 

 Devonian period. 



6. Upper Oolitic Series with inter stratified Porphyrinic Rocks. — 

 The sedimentary beds which here represent the Upper Oolitic system 

 are so interstratified with beds of eruptive porphyries, porphyritic 

 tuffs, and porphyry-conglomerates evidently contemporaneous, that it 

 is quite impossible to draw an} r line of demarcation between these 

 rocks ; and therefore I have followed the arrangement of Darwin in 

 Chile, in placing all the analogous rocks of Peru and Bolivia under 

 one head. Besides the above-mentioned porphyry-tuffs, conglome- 

 rates, and interstratified porphyries, we meet with claystones, mud- 

 stones, argillaceous shales and limestones, and other beds, many of 

 which bear a striking resemblance to the rocks of similar age in 

 Europe. In the south of the district here treated of, we find these 

 rocks abundantly fossiliferous : and the fossil shells from the beds of 

 the Desert of Atacama have yielded to the researches of MM. Bayle 

 and Coquandr, and Dr. Philippi, about thirty-three species of recog- 

 nized Oolitic forms. My collection from the same regions contains 

 a number not yet examined, and probably will yield further species. 

 In the part further north the country has been almost entirely unex- 

 plored, and the fossils obtained by me as yet have only been Litho- 

 trocJuis Andii, Ammonites DorneyMi, A. pustUifer, Oolitic Posidonice, 

 a Gryphcea,, and the cast of a Triyonia, as well as some vegetable re- 

 mains^ 



These beds are continuous from Chile right through the Desert of 

 Atacama; and, in combination with the fossil evidence, there can be* 



* " Die Algodon Bay in Bolivien," Denkschriften der k Akad. der Wissen- 

 schaft. Wien. Math.-Nat. CI. vol. iv. 1852. 



t Voyage dans TAmeriqiie Meridionale : Geologie, p. 07. 



J Mem. Soc. GrebL France, deux. ser. vol. iv. part. l e , 1851. 



§ I have not here considered it necessary to go into details as to the fossils, 

 both as my own collection from the Desert of Atacama has not yet arrived, 

 and because most of these more properly belong to Chile, and will be considered 

 in my next communication when treating of that country. 



