102 DE. J. W. EVANS ON THE [Feb. 1 894, 



and this in spite of the fact that, in the adjoining region to the 

 westward, the comparatively recent elevation of the Andes has 

 been accompanied by earth-movements of the greatest magnitude, 

 involving rocks of all ages. 



It is interesting to compare the geology of South America with 

 that of Southern Asia. There also enormous foldings have taken 

 place in connexion with the elevation of a great mountain-chain in 

 late geological times on the margin of an ancient land region, that 

 has since remote ages remained practically undisturbed, except by 

 the repeated intrusion and eruption of igneous rocks. This region 

 is, of course, peninsular India, which geologically presents many 

 points of resemblance to Brazil. Indeed, most of the formations of 

 the two countries may (to coin a new but self-explanatory phrase) 

 be considered ' tectonically homotaxial,' 1 or structurally equivalent, 

 to each other. Thus the ' Metamorphic ' corresponds to the ancient 

 crystalline rocks of Brazil. The ' Transition ' represents the Cuyaba 

 Slates ; while the Yindhyan represents the Corumba and Arara 

 Limestones and the Bizama Sandstone. The later Brazilian rocks 

 present points of resemblance to the Gondwana group. It is 

 doubtful whether the rocks I have compared are in any case con- 

 temporaneous with one another — in fact, their ages may be widely 

 different ; but they have taken a similar part in the building-up of 

 the present land, 



XI. Economic Peodtjcts. 



Gold has mainly been worked in the ferruginous conglomeratic 

 deposits overlying the Cuyaba Slates. These deposits have, through 

 a wide area, been completely turned over in the search for the 

 precious metal. They are now neglected, although a few streams, 

 such as the Coxipo, are occasionally worked. Most of the quartz- 

 veins that penetrate the slates are not very auriferous, but I was told 

 that some reefs gave good results. Gold is said also to be found 

 in the Chapada Sandstones, especially where there have been well- 

 marked breaks in the deposition. 2 



Diamonds. — These are found in deposits practically identical with 

 the alluvial workings in other parts of Brazil. They are chiefly 

 obtained from the valleys of two tributaries of the Paraguay, the 

 Bio Diamantino and the Bio Sant' Anna, and the tributaries of the 



1 [Two formations (in different countries of similar geological structure) will 

 therefore be ' tectonically homotaxial,' when they come into existence at the 

 same stage in the development of that structure, although they may not be 

 identical in actual age or fossil contents. — January, 1894.] 



- In a watercourse on the Chapada, north of Sant' Anna da Chapada, I 

 found a small fragment of rock, which was stated by a negro who accompanied 

 me to be a specimen of the rock from which gold and diamonds are obtained. 

 In colour it is greenish-grey, weathering to yellowish-red.. A great part of its 

 volume is made up of cavities, in shape and size like the grains of very fine 

 oolite. Most of the cavities are partly filled with crystalline quartz, and some 



