24 MK. J. A. DOUGLAS OK GEOLOGICAL SECTIONS [April 1914, 



have so wide a distribution, they vary little in general composition 

 throughout the whole area. They consist for the greater part of 

 porous white and pinkish rlrvofitic tuffs and ashes, usually con- 

 taining abundant quartz-crystals, and often much included pumice, 

 ranging from minute fragments up to blocks of considerable size. 

 The underlying rocks, too, are frequently visible along the sides of 

 the river-valleys, in the form of thick beds of porphyry-conglom- 

 erate, which Forbes included in the Jurassic System. No fossils 

 were obtained, however, from these beds and, as their field-relations 

 are usually most obscure, no exact determination can be made, 

 though I am of opinion that they will eventually prove to be of 

 Tertiary age. 



The question of the origin of the volcanic rocks is one of con- 

 siderable interest. Forbes 1 describes them as 



' having- been erupted through long narrow fissures or dykes, and poured out 

 over the country either as lava, or in some cases, as light volcanic ashes 

 emitted from the fissures, and deposited on the ground in their neighbourhood, 

 where they have gradually consolidated into beds.' 



Having spent considerable time in traversing this district, I have 

 come to the conclusion that such a theory of fissure-eruption does 

 not explain many of the observed phenomena. If these deposits 

 be regarded as true igneous lavas, their acid composition would in- 

 volve high viscosity ; and, for this reason alone, it would be hard to 

 account for the wide extension of individual sheets and the marked 

 horizontality of the bedding. Moreover, anything approaching 

 the nature of a lava-filled fissure is of rare occurrence. 



The general appearance of the beds is totally unlike that of any 

 true lava with which I am acquainted, and at once calls to mind a 

 deposit such as the well-known trass of the Broblthal in the 

 Eifel district. The only explanation, therefore, which to my mind 

 can account for their peculiar nature is that they have been formed 

 to a large extent as subaqueous deposits — an idea also partly 

 suggested by Forbes (o_p. supra cit. p. 25). 



The eastern portion of the Altaplanicie was, in comparatively 

 recent times, the site of a great system of lakes or inland seas (of 

 which Lake Titicaca alone survives), and- in mid-Tertiary times these 

 probably extended far to the westward, reaching to the foot of the 

 present Western Cordillera : being here well within the range of 

 volcanic activity, they became gradually filled up by ashes and 

 dust, with occasional pumiceous lavas. This hypothesis is further 

 supported by the fact that in many localities the volcanic beds are 

 interstratified with layers of fine gravel, which present the appearance 

 of lacustrine deposits and occasionally 3'ield organic remains. 



A few miles below the Mauri bridge were obtained specimens of 

 a silicified tree-trunk and a fragment of the symphysis of a 

 mandible of Ncsodon. The occurrence of the latter, 13,000 feet 

 above sea-level, is of considerable interest: for, as pointed out to me 

 by Dr. C. W. Andrews, it is almost identical in general appearance 



1 Q. J. G. S. vol. xvii (1861) p. 24. 



