Yol. 50.] GEOLOGY OF MATTO GR03SO. 91 



tain pebbles of older rocks. These are of all sizes, up to 8 inches or 

 more across. They are most plentiful in the north-west, and con- 

 sist mainly of granitoid rocks, in which shear-foliation was already 

 developed before they were included in their present matrix. Some 

 of the pebbles appear to be derived from clastic rocks, and a few 

 may be fragments of a compact lava-like rock. The pebbles are 

 sometimes isolated, sometimes a great number are found together ; 

 but each is embedded in the fine-grained matrix of the slate, the 

 divisional planes of which open out, so to speak, and envelop it. Iu 

 exposures in the channel of the Rio Jangada the slate contains 

 abundant fragments of these materials, which in some places make 

 up one half of the rock. Occasionally (as, for example, about half- 

 way between the Rio Espinheiro and the Rio Jangada) grit-beds of 

 similar materials, but in a finer state of division, occur interstratified 

 in the slate, their outcrops standing out like igneous dykes. Prof. 

 Eonney tells me that specimens of these conglomeratic rocks remind 

 him of some of the ashy beds in the porphyroid of Sharpley, 

 Charnwood. 1 



Castelnau found highly inclined slates with limestone on the Rio 

 Miranda, in South-eastern Matto Grosso ; see (2), l hTe Partie, vol. ii. 

 p. 466. 



3. Corumba' and Arara Limestones. 2 



I have placed these limestones together, though there is no clear 

 evidence that they are of the same age ; but there is considerable 

 resemblance between them. 



The limestone of Corumba is well shown in the cliff on which that 

 town stands. It occupies most of the lower portion of the extensive 

 island-like elevation, which at this point diverts eastward the course 

 of the Paraguay. 



1 Slate-conglomerates have been described from the Caxoeirinha do Eio 

 Pardo, Southern Bahia, (9) pp. 242-43, and from near Arrayas, Goyaz, ib. 

 p. 498. In the latter case the matrix is called by the observer gneiss, but 

 from the description it would appear to be slate. 



2 Limestones of more or less similar character are described as occurring 

 in the valley of the Hio Sao Francisco, especially on the eastern side. The 

 account given of the limestone of the Rio das Velhas (a tributary of the Sao 

 Francisco) reminds me of that of Arara, while farther north, near Chique-Chique, 

 the limestone would appear to more resemble that of Corumba. See (9), 

 pp. 278, 310-12, 327-28, 331, and (IS) p. 35. Similar limestone appears to be 

 found in the basins of the Jequitinhouha and Paraguassii in the States of Manas 

 Geraes and Bahia, (9) pp. 138, 302-3 ; in Goyaz (op. tit. p. 497), and also in the 

 Chiquitos (see infra, p. 96). I also found a limestone, somewhat like that which 

 occurs at Corumba, on the eastern bank of the river Paraguay, in the extreme 

 north of the republic of that name. 



According to Mr. Derby, " The specimens of limestone that have come to 

 hand from Corumba are strikingly similar in aspect to the limestones of Sao 

 Paulo and Parana that occur in a series that is certainly pre-Devonian, and is 

 presumed to be Cambrian or Lower Silurian. This presumption, however, has 

 for the present no firmer basis than the assumption, as yet unproven, that it is 

 older than a similar but apparently less metamorphosed series in the Sao 

 Francisco basin in which some obscure fossil corals, apparently of Upper 

 Silurian type, were found by the writer at Bom Jesus da Lapa," (8) p. 72. 



