Vol. 50.] GEOLOGY OF MATTO G ROSSO. 97 



Carboniferous ; and ho compares these different deposits with the 

 strata of the younger elevation of the Andes, in which later earth- 

 movements have produced the same consolidation and modification 

 as the earlier disturbances in the Brazilian. region had done long 

 before in its more ancient sedimentary deposits. A few isolated 

 exposures of Ilizama (Sandstone appear to occur in the alluvium 

 north and west of the Chiquitos ; see (1) pp. 184, 197, and 200-201. 



It may be suggested that the Ilizama Sandstone and Matto 

 Shales (unknown elsewhere in Brazil) are perhaps portions of the 

 Chapada Sandstones and accompanying shales, which have been 

 faulted down to their present position, outside and below the 

 Chapada escarpment. I have come to the contrary conclusion, for 

 the following reasons : — (a) The Chapada Sandstones are undis- 

 turbed and almost horizontal, within a comparatively short distance 

 of the region where the Ilizama Sandstone and Matto Shales have 

 been affected by considerable earth-movements, (b) These latter 

 rocks do not resemble the sandstones and shales of the Chapada. 

 The difference seems too great to be accounted for as the effect of 

 alteration, due to pressure accompanying disturbances, (c) In the 

 Chiquitos it seems clear, from A. d'Orbigny's account, that repre- 

 sentatives both of the Ilizama and Chapada Sandstones are found, 

 and that the latter are unconformable to the former. 



The question can only be absolutely settled either by fossil 

 evidence, or by following the Ilizama Sandstone and Matto Shales to 

 the north-east, and ascertaining whether they flatten out into the 

 Chapada Sandstones, or — a more probable supposition — are covered 

 unconformably by them. 



VI. — 7. Carboniferous (?) 



Argillaceous shales with fossil ferns occur at Miranda on the 

 river of the same name, one of the eastern tributaries of the Para- 

 guay. 1 These, Mr. Derby thinks, are probably of Carboniferous 

 age and correspond to fossil-bearing Carboniferous beds east of the 

 Parana, (8) pp. 66-67. He also thinks that rocks of the same age 

 will ultimately be found on the Chapada plateau ; see (8) p. 61. 



8. Trias (?) 



Above the shales is found, east of Miranda, a horizontally-bedded 

 sandstone with eruptive basalt-like rocks. 1 Mr. Derby believes this 

 to be identical with a similar sandstone exposed east of the Parana, 

 in which immense dykes and intercalations of augite-porphyrite 

 occur, a sandstone which is regarded provisionally as Triassic : see (8) 

 pp. 65-67. 



1 This information was obtained in the course of a survey of a proposed 

 railway from Curitiba, on one of the western tributaries of tbe Paraguay, to 

 Miranda, see (4). 



Q.J.G.S. No. 197. h 



