Vol. 62.~] THE ROCKS OF THE MADEIRA CATARACTS, ETC. 91 



are still imperfectly known ; and in the tropics the process of 

 lateritization often renders slates and crystalline rocks rich in 

 alumina almost indistinguishable from recently-formed alluvium. 

 It must be remembered, too, that there is a wide belt east of the 

 Andes which appears to have been depressed, and where the whole 

 country is flooded in the rainy season. In this region there must 

 be a considerable accumulation of alluvium, which may now cover 

 the ancient rocks of the ridge. 



We have, therefore, evidence of an important axis of 

 folding and elevation, with a south-easterly and north- 

 westerly direction, 1 in the centre of South America; 

 and there is reason to believe that it extends for a distance of 

 1200 miles, from 14|° lat. S. and 59° long. W. to 4° lat. S. and 73° 

 long. W. near Iquitos. Its direction is parallel to that of the Andes 

 in Northern Bolivia and Southern Peru, as well as to a small inter- 

 mediate axis of elevation in Chiquitos in South-Eastern Bolivia. 2 



These lines of folding follow the north-westerly and south- 

 easterly strike, which is so common on the earth's surface for 

 some distance on either side of the Equator. It also appears to 

 prevail in Southern Venezuela, in Guiana, and in North-Eastern 

 Brazil, where a barrier is believed by Dr. Eriedrich Katzer 3 to have 

 formerly stretched across the site of the mouth of the Amazon. 



There appear to be four principal directions of folding or elevation 

 in South America, namely : 



(1) The north-westerly and south-easterly strike already described, which is 

 also met with in the south of the continent in Argentina and Patagonia, as 

 well as in Tierra del Fuego and the Falkland Islands. 



(2) A direction roughly at right-angles to the former, approximately north- 

 east and south-west, following the coast of Brazil south of Cape San Eoque. 

 It also occurs on the Upper Paraguay aud the Xingii,. and in the Colombian 

 Andes. 



(3) An east-and-west strike seen in Northern Venezuela and Trinidad, and in 

 the Amazonian basin, which consists broadly of a syncline bounded by two lines 

 of elevation on the north and south. 



(4) The north-and-south line of the Andes, from Southern Bolivia to 

 Patagonia. 



These directions are roughly indicated in the sketch-map which 

 forms fig. 1 (p. 90). In the present imperfect state of our know- 

 ledge of the geology of South America, it is impossible to give their 

 exact positions. 



The movements along all these lines appear to have extended 

 over a considerable period. The north-easterly and south-westerly 

 folding of Matto Grosso was mainly pre-Devonian, while the 

 north-westerly and south-easterly movements in the Andes ex- 

 tended to at least post-Carboniferous times. The north-and-south 



1 More exactly north 55° west. 



2 A. d'Orbigny, ' Voyage dans l'Amerique Meridionale, execute psndant les 

 annees 1826-33' vol. iii, pt. 3 (Geologie), Paris 1842, pp. 183-99; and 

 J. W. Evans. ' The Geologv of Matto Grosso ' Quart. Journ. Geol. Soc. vol. 1 

 (1894) p. 1)6. 



3 ' Grundziige der Geologie des Unteren Amazonasgebietes' Leipzig, 1903 

 p. 240. 



