88 THE GEOLOGY OE MATTO GKOSSO. [Feb. 1 894. 



sudden descent to a lower, but still hilly region usually less than 

 300 metres (about 975 feet) above tbe sea. 1 



Farther south this hilly country is replaced by extensive marshy 

 plains continuous with the lowlands of Bolivia, the Paraguayan 

 Chaco, and the Argentine plains, and extending with but little in- 

 crease of elevation to the Bolivian tributaries of the Amazonas. 

 From these alluvial tracts rise here and there a number of rock- 

 masses of various dimensions, from the extensive highlands of the 

 Chiquitos to isolated eminences of insignificant proportions. 2 But 

 in the east the high land of the Chapada continues southward 

 and connects with the hills of Paraguay. 



The majority of the rocks in Matto Grosso may be tabulated in 

 the following manner. The correlation of strata in different locali- 

 ties must, in the almost entire absence of fossils, depend to a large 

 extent on lithological characters — an unsatisfactory basis, but all 

 that is at present available. I am responsible for the names of 

 Nos. 2 to 5 ; I have not seen rocks 7 to 9. 



r» . „ in [Alluvial deposits. 



Quaternary 10.i TJ - , -, / ? -, ., 



(High-level suriace-deposits. 



Cretaceous? 9. Sandstones of the Taboleiros. 



Trias? 8. Sandstone, with igneous rocks, near Miranda. 



Carboniferous? ... 7. Shales, with fossil ferns, near Miranda. 



Devonian 6. Chapada Sandstones and Shales. 



(Probable unconformity.) 

 ' 5. Matto Shales. 



(Relations not shown.) 

 4. Rizama Sandstone. 



(Perhaps some unconformity.) 



Pee-Devonian ■{ 3. Corumba and Arara Limestones. 



/A , r .,.« (Very marked unconformity.) 



(As yet uniossililerous L n v , . ; «, . J ' 



v -,' ,. , N 2. Cuvaba Slates. 



and ot unknown age.) i a , r ., N 



b ' | (Strong unconformity.) 



( 1. Ancient crystalline rocks. 



IY. Pre-Devonian Rocks. 



1. Ancient Crystalline Rocks. 



These occur in situ at TJrucum, near Corumba. Pebbles of 

 similar rocks are found at various points in the Cuyaba Slates. The 

 Uruciim rocks are foliated and schistose in appearance ; though 

 Prof. Bonney, who kindly examined specimens and thin sections of 

 them, thinks that they are really of .igneous origin, but have been 

 crushed and sheared so as to take up their present structure. Some 



1 Gauss, from daily barometric readings extending over about two months, 

 calculated the height of Cuyaba as 201 metres or 653 feet, (6) p. 168. My 

 aneroid observations, made at Santa Cruz (Barra dos Bugres) on the Rio Para- 

 guay, about 15° S. lat., show the Paraguay there to be a little higher than the 

 Rio Cuyaba at Cuyaba. I found the foot of the Chapada north-west of Cuyaba 

 to be at a height of about 320 metres or 1040 feet above sea-level. 



2 The height of the Paraguay at Corumba, and therefore practically of the 

 low plain adjoining, appears to be about 120 metres or 400 feet : see (5) p. 22. 



