Vol. 62.] CATARACTS OP THE RIVER MADEIRA, ETC. 123 



south of the Amazonian syncline. No doubt these rocks would be 

 found below the alluvium of the Lower Madeira. 



IV. Summary. 



The crystalline rocks of the cataracts of the River Madeira and 

 the lower waters of its tributaries are part of a ridge, with a north- 

 westerly and south-easterly strike similar to that of the Andes in 

 the same latitudes. This strike, which is especially prevalent in 

 Equatorial regions, is probably due to the same causes as those that 

 have resulted in the eastern position of the land-masses of the 

 Southern Hemisphere as compared with the Northern. 



With the exception of comparatively-recent alluvial deposits, 

 and a few pebbles of chert of marine origin but uncertain date, 

 only crystalline rocks are met with. They all appear to be igneous, 

 mostly massive in character, though some dyke-rocks occur. In 

 places they are typical gneisses, and they are often banded; but 

 in some cases they show no signs of foliation. The prevailing type 

 is acidic, with a considerable proportion of alkalies, especially soda ; 

 but some of the dyke-rocks are distinctly basic in character. The 

 more acid rocks are usually fine in grain, and are often granulitic 

 in structure. In most cases the quartz seems to have crystallized 

 out before the felspar. The causes of these characters are briefly 

 discussed. 



The occurrence of andalusite of a cbiastolite-type as an inclusion 

 in a felspar is noted, as well as an unusual type of allanite. An 

 altered basalt is described, containing minute concentric structures 

 allied to those of a pyromeride. 



Above and below the region of the cataracts is a wide expanse of 

 alluvial country, either of recent or later Tertiary date. 



In conclusion I wish to express my obligations to Prof. Bonney, 

 Sc.D., F.R.S., who kindly examined many of the rock-sections ; also 

 to Dr. G-. T. Prior, of the British Museum (Natural History), and 

 others, for criticism and suggestions during the preparation of this 

 paper. I must also acknowledge my indebtedness to Mr. Thomas 

 Crook, F.G.S., for valuable assistance in photographing the micro- 

 scope-sections shown in Plate V. 



EXPLANATION OF PLATE V. 



Fig. 1. Section of granulitic gneiss from Lages, Rio Madeira, above the con- 

 fluence with the Beni ; magnified 21 diameters. Ordinai*y light ; 

 showing acid plagioclase, quartz in blebs or rounded idiomorphic 

 crystals, biotite, and micrographic structure. (See p. 9§.) 

 2. The same in polarized light, between crossed nicols. The plagioclase 

 is seen to contain inclusions of orthoclase and quartz. 



