92 dr. j. w. evans on the [Feb. 1894, 



The limestone is, when purest, dark blue in colour. It passes into 

 paler and more argillaceous beds, and sometimes into a yellowish 

 calcareous shale : it is often siliceous. The dip is usually from 10° 

 to 15° north, but frequently near tbe margin changes, so as to be 

 directed towards the alluvial plain. 



This limestone is also found at low elevations, rising out of the 

 plain north and east of Corumba, as, for example, at Castelhos and 

 near Carandazinho l on the Rio Paraguay. 



Behind Corumba there occurs in the limestone a thick deposit of 

 ferruginous chert. Its superior hardness gives rise to a long ridge 

 at the back of the town some hundred metres high. 2 Near Corumba 

 the limestone is sometimes brecciated and re-cemented, and I found 

 specimens showing this re-cemented material and also the normal 

 rock silicified into a kind of flint. 



At Coimbra is found an impure limestone, often yellowish or red 

 from the presence of iron. Some varieties appear to be dolomitic, 

 as they only effervesce with hot hydrochloric acid. I paid a short 

 visit to a large cavern in this limestone about 5 or kilometres 

 (3 to 3f miles) north of Coimbra. It has been often described ; see 

 (#), l fere Partie, vol. ii. p. 406, and (5) vol. i. pp. 271-285. It has 

 extensive ramifications and numerous stalactites ; the floor is gene- 

 rally composed of a ferruginous sand, the residue left after the 

 removal of the carbonate of lime from the limestone. 



The Arara Limestone occurs in the parallel ranges of hills already 

 referred to (p. 90) between the upper course of the Paraguay and 

 the llio Cuyaba, especially at Arara, an isolated hill at the northern 

 end of the most easterly ridge. It is also found in the adjoining 

 range to the westward. South-west of Chapedon, in the valley 

 beyond, is an isolated natural turret of limestone, the rest having 

 been removed by denudation. 



This limestone is pale and streaky, rather more compact and 

 altered than that of Corumba. Close to Arara it usually dips at 

 about 15° to the south-west, but is in some places much contorted. 

 Farther west it is nearly horizontal, while the Cuyaba Slates have a 

 steep dip. As a result of weathering, exposed surfaces of the Arara 

 Limestone become studded with acicular points. 3 



I was unable to find any organic remains in the Corumba or 

 Arara Limestones ; but I was told at Sao Luiz de Carceres that 

 limestone containing shells is found in the hills east of that town, 

 viz. the southern extension of the hills in which Arara Limestone 

 occurs. It is, of course, possible that these shells are land-mollusca, 

 embedded in travertine of comparatively recent formation. 



1 Near this locality I noticed a large stream flowing into a low cave, at the 

 base of a limestone-hill. 



2 Compare the chert stated to occur in the neighbourhood of Volta da Serra, 

 near Jacobiua, in the State of Eahia. It is there called jpedra de fogo or fire- 

 stone, (9) p. 312. 



3 The same effect of weathering has been noted at Chique-Cbique ; see (9) 

 p. 310. 



