-'2 DR. J. W. EVANS ON THE HOCKS OF THE [Feb. I906, 



and east-and-west lines of movement appear to be of Tertiary and 

 Quaternary age. Along the Pacific coast, movements with lines of 

 strike parallel to the shore-line have, it need scarcely be said, 

 continued down to historical times. 



II. Previous Work. 



With the exception of some brief notes by Joseph and Franz 

 Keller and Col. Church, 1 the only description which has yet been 

 published of the rocks of the cataracts is that given by Dr. Joao 

 Severiano da Fonseca, who in the year 1877 descended the rivers 

 Guapore, Mam ore, and Madeira with the Commission for the 

 delimitation of the frontier between Brazil and Bolivia. 2 As this 

 book is comparatively inaccessible to geologists in this country, I 

 have translated, and reproduce here, those portions which seemed 

 to be of interest in connection with this paper. 



The following paragraphs refer to the geology of the cataracts as 

 a whole : — 



Vol. ii, p. 280. ' The rocks of these cataracts are of plutonic formation, and 

 reveal at the first glance their Tolcanic origin, modified perhaps by meta- 

 morphism. Some were difficult for me to classify, on account of the obscurity 

 of their characters ; in others the mineralogical facies was satisfactorily deter- 

 minable. The great trachy tic pavements, which are nearly smooth, and either 

 of a ferruginous hue or shining black like pitch, are formed in many places of 

 superposed beds, which are more 01* less undulating and have curvilinear 

 borders, as if they had been derived from viscid melted material poured 

 forth in great outbursts, and forming sheets of which the later solidified before 

 they reached the distance to which the earlier had extended. Here and there 

 appeared large rock-masses, some prismatic in shape, others rounded : in one 

 place were dykes of diorite and of elvan ; in another loose blocks. Some were 

 split in the middle by a mere crack, others by a gap of more than a fathom in 

 width.' 



' There were likewise large cauldrons, holes in the pavement, perfectly 

 rounded, the formation of which is easily explained by the attrition of stones 

 rolled about in small depressions whicb, little by little, with the movement 

 of the waters and the passage of centuries, become larger and more rounded. 

 But it is not so easy to explain the elliptical holes in some of these pave- 

 ments .... all are of the same dimensions, and as if arranged in uniform 

 directions one after the other in two or three lines, so that they call to 

 memory, though without any resemblance, human footsteps. The most 

 notable are those of the cataracts of the Madeira, Bananeira, Ribeirao, and 

 Paredao. Their dimensions are : 1 to 3 decimetres long, a third more or less 

 of this broad, and nearly as much deep, always preserving an ellipsoidal form. 

 Are they spaces formerly occupied by bodies easily disintegrated or decomposed 

 by the waters, and that in time became vacuous ? As for the pavements, in 

 spite of their being varnished by the attrition of the water and brilliant 

 with a black metallic polish, it is not difficult to classify them by their texture 

 and system of agglutination. They are hornblende-porphyries, obsidians, 

 syenites, petrosilcx, etc. — all felspathic rocks. The canga (ferruginous con- 

 glomerate) appears in lofty crags of a reddish-black colour, whence they have 

 received the Tupic name tupanhona canga. At the same time, dykes of 



1 Franz Keller-Leuzinger, ' Vom Amazonas & Madeira ' Stuttgart, 1874 ; 

 and George Earl Church, ' The Route to Bolivia via the River Amazon (a Report 

 to the Governments of Bolivia & Brazil) ' London, 1877, p. 186. 



2 ' Viagem ao Redor do Brasil ' Rio de Janeiro, 1880-81. 



