52 GEOLOGICAL SECTIONS THROUGH [April 1914, 



railway works, and the period which had elapsed since Forbes 

 visited the country had witnessed the development of microscopical 

 petrology. Of these advantages the Author appeared to have 

 availed himself to the fullest extent. 



The speaker shared the belief which the Author had expressed 

 as to the former greater area of Lake Titicaca. He had traversed 

 plains to the north of the lake which bore the appearance of having 

 been recently covered by a shallow extension of its waters. He 

 could not, however, agree with the Author that the granite of the 

 Eastern Cordillera was probably of Carboniferous age — where he 

 had seen it in the Tipuani Valley, it showed no signs of alteration 

 by pressure ; while the fact that an outlier of Carboniferous rocks 

 occurred in a sharp synclinal fold near Nube on the Rio Kaka, a 

 little to the east of the Cordillera, indicated that the rocks were 

 subjected to powerful folding movements after Carboniferous times. 



Dr. A. YatjghajS t joined in complimenting the Author upon his. 

 most interesting paper, and testified to the very great labour entailed 

 in the examination of so large a collection and to the wide know- 

 ledge of every branch of geology which it demanded. In view 

 of the fact that here, as in other parts of South America, the 

 stratified series represented almost entirely the deposits formed 

 during the great transgressions, the speaker asked whether the age 

 of the Devonian rocks, in the section just described, was the same 

 as that of the Argentine Devonian (the rocks described by Dr. Ivor 

 Thomas) ; and, furthermore, what was the evidence that this date 

 was Lower Devonian rather than Hercynian : that was, transitional 

 between Lower and Middle Devonian ? 



Dr. L. L. Fermob, said that, in the transcontinental and Yukon- 

 Alaska excursions of the recent International Geological Congress, 

 he had had several opportunities of examining the granodiorites of 

 the Canadian section of the Cordilleran ranges, and had become 

 interested in Prof. Daly's views on the magmatic stoping and assimi- 

 lation of the intruded rocks by the granodiorites of Canada. He 

 drew attention to the distinction that the Author had made between 

 the modes of intrusion of the granodiorites and diorites mentioned 

 in his paper. The granodiorites had reached their present position 

 by replacement, and the diorites by displacement, of the associated 

 rocks. Intrusion by replacement seemed, to the speaker, to neces- 

 sitate incorporation of the replaced rock in the intruding magma. 

 He asked the Author what evidence he had of the assimilation of 

 the invaded rocks by the granodiorites, and whether he considered 

 that the acid character of the granodiorite (when compared with 

 the diorite) could be explained on that basis. 



Mr. E,. B. Newton referred to some brachiopod remains collected 

 some years ago by Sir Martin Conway while crossing from Mount 

 Milluni to the Huaina-Potosi Mine, at an elevation of 16,500 feet. 

 In reporting upon these specimens, the speaker mentioned the 

 occurrence of Anoplotheca Jlabellites as a characteristic Lower 

 Devonian form, known also from the Oriskany Sandstone of the 

 United States, and from rocks of similar age in South America, 



