Vol. 70.] THROUGH THE A^DES OF PERU AND BOLIVIA. 7 



light of recent research on the distribution of igneous rocks, 

 I have decided to make at some future date a more detailed 

 study of the igneous rocks of Peru taken as a whole. 



For the purposes of description, this first or southern seotion 

 may be conveniently divided into three parts : — 



(i) The Mesozoic sediments of the coastal region with their con- 

 temporaneous igneous rocks, the intruded core of gTanodiorite, 

 and the overlying recent volcanic deposits of the Western 

 Cordillera. 



(ii) The volcanic beds of the Mauri River, the Mesozoic and Palaeozoic 

 rocks of the Altaplanicie and Titicaca district, the line of 

 dioritic intrusions, and the Pleistocene deposits of the Des- 

 aguadero River. 



(iii) The Palaeozoic rocks and granitic core of the Eastern Cordillera 

 and the Amazon slopes. 



(i) The Geological Structure of the Coastal Region and 

 the Western Cordillera. 



(a) The Mesozoic deposits and the contemporaneous 



igneous rocks. 



A magnificent section of stratified Mesozoic rocks is exposed 

 along the whole coast of Northern Chile, the cliffs rising 

 abruptly from the shore and being plainly visible from the 

 coasting steamers. They terminate northwards in the Morro 

 de Arica, a steep hill south of the port of that name, beyond 

 which the ocean is bordered by a broad strip of coastal desert. 

 This hill forms the starting-point of our first transverse section 

 through the Andes. 



The bulk of the Morro is made up of volcanic rocks, with certain 

 stratified beds appearing at the base and towards the summit, the 

 true age of which has always been a matter of some doubt. They 

 were originally coloured as Carboniferous by A. d'Orbigny, 1 from 

 the supposed occurrence of JProductits in limestone included in 

 the lava. Forbes, 1 though apparently obtaining no palajontological 

 evidence, classes them tentatively with the Upper Oolitic Series, 

 with the following comment : — 



■ This evidence [of A. d'Orbigny] does not appear to me sufficiently con- 

 clusive to warrant its being separated from the other strata, which appear 

 continuous and which are decidedly of Upper Oolitic age, — more particularly 

 as we have no example of the occurrence of Carboniferous beds anywhere 

 along the coast of the Pacific in South America.' (Q. J. G. S. vol. xvii, 1861, 

 p. 51.) 



Though the latter statement will be shown in a future paper to 

 be incorrect, the conclusion arrived at was more or less accurate. 



Prof. E. Suess 2 suggests a comparison of the Morro de Arica 

 with the Cretaceous mountain -zone of Tierra del Fuego. which is 

 said in places to assume a Palaeozoic aspect. 



In 1909 Senor Escutti Orrego, 1 of Arica, published a short 



1 See Bibliography, § VI, p. 49. 



- ' The Face of the Earth ' Engl, transl. vol. i (1904) pt. 2, chapt. ix, p. 527. 



