10 ANNIVEKSART ADDEESS. 



Mr. Nicolay shows that the principal bottom rocks are Palaeo- 

 zoic, chiefly gneiss ; passing, on the one hand, into porphyry and 

 granite, and, on the other, into hornblende rock and quartzite. 

 They are surmounted near the coast by conglomerates and schists, 

 and in other parts by sandstones, a tertiary conglomerate, or 

 sandstone, forming the Chapada and other horizontal surfaces 

 to which that name is given. The general strike is N.N.E. to 



s.s.w. 



The area includes not only that of his map, but also some 

 portion of the province of Piauhy, to which his explorations 

 extended. 



The beds described all cross the province of Bahia. 



The Chapada is 200 miles from the western edge of the Lagos 

 or bay, attaining an elevation of 3,000 feet, and carrying the 

 diamond workings to the summit. 



The sandstones are more transmuted there than near the coast. 

 In the latter district they are rich in diamonds, but the works 

 have been abandoned. 



In ascending order we have the following geological series : — 



1. Primary rocks. 



2. Schist. 



3. Limestone. 



4. Sandstone. 



5. Quartzite (an altered rock). 



6. Conglomerate. 



These are traversed by dykes of trap and chert, with veins 

 filled in with earth, sand, and clay. 



In the Cascalho or diamond beds are found the following 

 minerals : — 



1. Crystals of iron pyrites, derived from quartzites — the 



latter without diamonds. 



2. Magnetic iron. 



3. Oxide of iron (JPedra deferro). 



