122 DR. J. W. EVANS ON THE ROCKS OP THE [Feb. I906, 



In the latter case the role of the organic material would be confined 

 to preventing the further oxidation of the manganese and iron, 

 and consequent separation of carbonic acid. Ultimately, on the 

 evaporation of the water on the surface of the rocks, this oxidation 

 would take place, and the black crust would be formed. Such 

 evaporation w T ould obviously be of most frequent occurrence near 

 cataracts in hot countries, where the rocks are repeatedly moistened 

 by the waves and spray, and dried by the tropical sun. It is 

 remarkable that in none of the analyses of the water of the Lower 

 Amazon and its tributaries, given by Dr. Katzer, is there any 

 mention of the presence of manganese. 1 Some interesting man- 

 ganese-deposits on low ground are described (pp. 95-98), and may 

 have an origin similar to that of the black coating on the rocks of 

 the cataracts. 



Since the above was written a valuable contribution to the subject 

 has been made by Mr. A. Lucas, Chief Chemist, Survey-Department 

 Laboratory, Cairo, 2 who has examined a large number of similar 

 deposits on the rocks of the Nile Cataracts, as well as other feebler 

 films on desert-roi-ks. He inclines to the belief that such deposits 

 are an efflorescence from the rocks immediately beneath. This is 

 an opinion to which I am unable to subscribe. Not only have I 

 found large crystals of quartz and felspar coated equally with 

 biotites or hornblendes, but I have seen in the Upper Paraguay 

 crags of white sandstone covered externally by a jet-black coating 

 which caused them to resemble basalt. 



(7) Below the Cataracts. 



The rocks of the island below Santo Antonio are the last 

 crystalline rocks on the Rio Madeira. From this point in lat. 9° S. 

 no rocks are exposed in the river-bed or banks, except argillaceous 

 deposits usually more or less ferruginous, but sometimes nearly 

 pure white. These may form banks 40 or 50 feet high at low 

 water, but I saw nothing to lead me to suppose that they are 

 the result of the alteration in situ of hard rocks. The Neogene 

 deposits of Dr. Katzer may, however, be represented. 



Three or four degrees farther to the eastward on the Tapajos, 

 diorite and other crystalline rocks are said to occur. 3 Their 

 presence on the Xingu, still farther to the east, has already been 

 mentioned (p. 88). Farther north Palaeozoic rocks are met with, 

 extending westward to the margin of the basin of the Madeira 

 between 5° and 6° lat. S., 4 following the line of elevation on the 



1 ' Grundziige der Geologie des Unteren Arnazonasgebietes ' 1903, p. 48. 



3 ' The Blackened Rocks of the Nile Cataracts & of the Egyptian Deserts ' 

 Cairo, 1905. 



8 F. Katzer, op. supra cit. pp. 234 & 236. 



4 Ibid. p. 170. 



