88 DB. J. W. EVANS ON THE ROCKS OF THE [Feb. 1906, 



The Rocks of the Cataeacts of the River Madeira and the 

 Adjoining Portions of the Beni and Mam ore. By John 

 William Evans, D.Sc, LL.B., F.G.S. (Read November 22nd, 

 1905.) 



[Plate Y — Microscope-Sections.] 



Contests. 



Page 



I. General Configuration of the Region 88 



II. Previous Work 92 



III. Geological and Penological Observations : — 



( 1 ) Between the Andes and the Cataracts 93 



(2) The Cataract of Esperanza on the Beni 95 



(3) The Cataracts of the Mamore 98 



(4) The Cataracts of the Madeira 99 



(5) The Granulites and their Genesis 119 



(6) The Black Coating of the Eocks of the Cataracts ... 121 



(7) Below the Cataracts 122 



IV. Summary 123 



I. General Configuration or the Region. 



More than eleven years have passed since the publication of a paper 

 by this Society on ' The Geology of Matto Grosso,' x in which I 

 described tbe rocks that I met with in the area drained by the 

 Upper Paraguay and its tributaries. They include granitoid gneiss, 

 highly-cleaved slates, limestones, sandstones, and shales, and have 

 a prevailing north-easterly and south-westerly strike parallel to the 

 coast-ranges of Southern Brazil. To the northward they are covered 

 unconformably by a younger series of sandstones and shales con- 

 taining imperfectly-preserved Devonian fossils. 2 These dip gently 

 northward, and form a broad undulating tableland extending from 

 east to west. Still farther north, between latitudes 10° and 4° S. 

 on the Xingu (pronounced Shingu), Dr. Karl von den Steinen 3 

 found granite, gneiss, and other crystalline rocks. Nothing is said 

 about the strike, but it seems that the trend of the hills was 

 approximately south-west and north-east, as in the basin of the 

 Upper Paraguay. 



East of the River Paraguay the high ground changes its trend, 

 and stretches in a north-westerly direction to the cataracts of the 

 Mamore and Madeira, under the names of the Serra dos Parecis and 



1 Quart. Journ. Geol. Soc. toI. 1 (1894) p. 85. 



2 O. A. Derby, ' Nota sobre a Geologia e Paleontologia do Matto Grosso ' 

 Archivos do Museu National do Bio de Janeiro, vol. ix (1890) pp. 59-88. 

 Mr. Derby believes that the two series of sandstones and shales are identical ; 

 but, although I nowhere saw them in contact, they appeared to be in every 

 way distinct. 



3 ' Durch Central-Brasilien ' Leipzig, 1886, pp. 330 (227), 333 (268), and 

 maps III & general. 



