00 GEOLOGICAL SECTIONS THROUGH [vol. lxxvi, 



DlSCUSSIOX. 



Dr. J. W. Evans congratulated the Author on the excellent work 

 that he had accomplished. The ground that the speaker had the 

 opportunity of examining in the Eastern Andes lay between 

 the sections described by the Author in his two communications 

 to the Society. It differed, in some respects, geologically from the 

 region that had been dealt with on the present occasion. The Car- 

 boniferous formation occurred in the first and third synclines 

 (counting from the west) in the Palaeozoic rocks, but not in the 

 second. Farther east the latter had been thrown into isoclinal 

 fplds, which included beds of Tremadoc and Llandeilo age, and 

 possibly pre-Cambrian strata, but no crystalline schists ; nor were 

 there (with perhaps one unimportant exception) any soda-rocks. 

 The low ground between the Palaeozoic ridges was occupied by red 

 rocks believed to be of Cretaceous age. 



Prof. Gr. S. Boulgee, having some years ago traversed the line 

 of the Author's section throughout, thanked him for his admirable 

 description of the geological structure of the country. He 

 enquired as to the source of the lateritic deposits in the river- valleys 

 at the eastern foot of the Andes. 



Mr. W. Campbell Smith asked whether the Author had any 

 evidence, other than the presence of the quartz-grains, to support 

 his suggestion that the dioritic intrusion, shown in the section west 

 of Taya Taya. had absorbed the Jurassic quartzite through which 

 it was intruded, and that the quartz-grains were the relics thereof. 

 The suggestion involved the acceptance of the ' stoping ' and 

 almost complete solution of a quartzite by a dioritic intrusion. 

 This seemed an important point, on which further evidence was 

 desirable. 



Prof. W. J. Sollas also spoke. 



The Author, in reply to Dr. Evans, said that he was himself 

 convinced that the mica-schists of the San Graban valley were 

 largely the result of d} r namic nietamorphism, for the associated 

 igneous rocks furnished clear proof of intense crushing. He did not 

 claim, however, that this metamorphic zone was of wide lateral 

 extent, and he had not met with it in the district east of La Paz. 

 He was not surprised, therefore, to hear from Dr. Evans that it 

 did not occur in the Caupolican area. This fact appeared to the 

 speaker to lend strength to his suggestion that these ancient rocks 

 had been instrumental in checking the advance of the folds into 

 the Brazilian platform : for in the south, where they were absent, 

 Dr. Evans had described subsidiary forefolds, and the Carboniferous 

 transgression was of wider extent. 



He agreed with Prof. Boulger that much of the soil in the 

 forested region Avas due to lateritization ; he had found extensive 

 deposits of laterite in the Chanchomayo district, and regarded it 

 as the normal product of weathering in these latitudes. 



