Vol. 50.] GE0L0GT OF MATTO GR0SS0. 101 



rock, except that it is probably younger than the Matto Shales. T 

 had, unfortunately, no opportunity of determining whether it 

 occurred as a dyke or as the outcrop of an intrusive or interstratified 

 rock. In connexion with this basalt, Mr. Derby writes to me : — 

 " Dykes, often of considerable size, of similar rocks are common in 

 the Devonian and Carboniferous regions of the Amazonas and of 

 Sao Paulo and Parana, as well as in the regions characterized by 

 the older rocks." 



X., Historical Summary. 



It may not be out of place here to briefly detail the succession of 

 conditions indicated by the geological structure of Matto Grosso 

 and the surrounding regions. 



At a remote period the materials now forming the Cuyaba Slates 

 were deposited over an extended area. In some places we have 

 indications of the neighbourhood of land, consisting mainly of the, 

 even then, ancient crystalline rocks, which had already assumed the 

 characters that they now possess. Then came a period of great 

 earth-movements resulting in the folding, cleavage, upheaval, and 

 denudation of the slates. Subsequently, after depression had 

 occurred, the Corumba and Arara Limestones were widely deposited 

 on the crystalline rocks and upturned edges of the Cuyaba Slates. 

 Thenceforward there is progress to continental conditions. The 

 Kizama Sandstones and Matto Shales were deposited in successively 

 diminished areas, perhaps in freshwater basins. 1 A second series 

 of movements (which had probably commenced at the close of the 

 deposition of the limestones) now moulded all the then existing 

 rocks into the continental mass that still forms the framework of 

 the Brazilian highlands. As none of these rocks yield fossils, we 

 can say nothing concerning the period when these events took place. 

 In Silurian times we find the sea covering the region west of Brazil, 

 where the Andes were subsequently uplifted, and also occupying the 

 Amazonas depression between the large islands now represented by 

 Guiana and Central Brazil : see (10) p. 160. In the Devonian period 

 general depression prevailed, during which extensive deposits were 

 laid down on the denuded surface of the old rocks. The depression 

 continued to some extent during the Carboniferous period; The 

 land then again rose, and since that time land conditions have 

 mainly prevailed. The sandstones of the Taboleiros and similar 

 rocks in other parts of Brazil seem to be local freshwater deposits ; 

 it is only on the sea-border that marine Secondary strata are 

 found. From the Devonian period to the present time the horizon- 

 tality of the rocks seems to have been (except at a few points on 

 the margin of the continental massif) almost entirely undisturbed ; 



1 The Matto Shales especially have the appearance of being of freshwater 

 origin. 



