572 Rejjorts & Proceedings — Geological Society of London. 



simple individuals are rare. Groups of crossed crystals are often met 

 -with which are not true twins. The various occurrences — in eruptive 

 rocks, metalliferous veins, quartz veins, and sedimentary rocks — are 

 described in detail. The paper is amply illustrated. 



11. Fluting and Pitting of Granites. — An interesting and well- 

 illustrated article on this subject, by Mr. J. C. Branner, has been 

 published by the American Philosophical Society (Proc. lii, April, 

 1913). The fluting of granites and other crystalline rocks appears 

 to be confined to tropical and possibly sub-tropical countries. Striking 

 examples are seen near Quixada, in Ceara State, Brazil, mostly in 

 massive coarse-grained gneissoid granodiorites. The furrows start 

 at or near the summit of the exposed rock, and run straight down 

 the rock-slopes by the shortest possible routes. Some reach a depth 

 of nearly 2 metres measured at right angles to the general surface 

 of the rock-masses. In the same region the fluted rocks have been 

 hollowed into " great rounded caldron-like pits, some of which are 

 associated directly or indirectly with the fluting ". These pits seldom 

 exceed 2 metres in depth and their diameter is about 2 metres. 

 In certain cases they occur in a nearly vertical row, connected by 

 a furrow, and having the appearance of a great irregular staircase 

 mounting the hill. The fluting seems to -occur only on steep slopes, 

 with an angle of 45° or more, and it is caused by the rainfall, small 

 in amount, that acts in part chemically, in part mechanically. The 

 pits are formed by the disintegration and dissolution of minerals. 



I5.E1FOI4TS .A-lsTID :PE,OOElEIDIlsrGi-S. 



I. — Geological Society of London. 



November 5, 1913.— Dr. Aubrey Strahan, F.R.S., President, 

 in the Chair. 



The following communication was read : — 



" Geological Sections through the Andes of Peru and Bolivia." By 

 James Archibald Douglas, M.A., B.Sc, F.G.S. 



This paper deals with the geological structure of the South 

 American Andes, as illustrated by a horizontal section drawn from 

 the port of Arica in the extreme north of Chile (formerly Peruvian 

 territory) across the mountain-ranges or 'Cordilleras' to the forested 

 region of the Amazon slopes, in the district known as the Bolivian 

 'Yungas', following the route of the new Arica- La Paz railway, 

 which was under course of construction at the time of the author's visit. 



It is the partial result of two vears' geological exploration in Peru, 

 undertaken on behalf of Mr. W". E. Balston, F.G.S., for the Oxford 

 University Museum. After a description of the general physiography 

 of the Peruvian Andes, the topographical features of the country 

 traversed by the railway are discussed in some detail. 



Its geological structure is then described under three headings : 

 (1) The Mesozoic sediments of the coastal region with their contem- 

 poraneous igneous rocks, the intruded core of granodiorite, and the 



