AJSTNIVEESAEY ADDEESS. 11 



4. Black silica {Pedra defogo). 



5. Hyaline quartz (^Pingua d^agoa). 



6. Hydrophosphate of alumina {^Fahras and Gahocles). 



7. Schorl {Fejoes or Feijao). 



8. Tttria. 



The colours of the diamonds are various, changing with the 

 distinctive localities, but the characteristic colour is green. 



According to the published lists and the testimony of Mr. 

 Hartt and others, the gneissic rocks and older clay slates in 

 Brazil are of about the same age as the Laurentian formation of 

 North America, with a probability that some of the mica schists 

 are of Lower Silurian age. In Bahia province there are also some 

 dioritic gneiss and syenite. 



Itacolumite, of which we have heard on a former occasion, is a 

 sort of bugbear to geologists ; it seems to be, after all, only a 

 lower Palseozoic rock. 



In Bahia no Devonian rocks have been ascertained, nor is it 

 certain that carboniferous rocks exist in that province, if, indeed, 

 at all north of the tropic. But rocks referable to Cretaceous are 

 common a little south of Bahia (the bay), and are found at 

 intervals in the provinces to the north. The group at Bahia is 

 considered to be Neocomian. Of tertiary rocks there is a great 

 abundance in Bahia, and we have already seen that the diamonds 

 of that province belong to sandstones of that age. 



There is also a vast amount of conglomerated drift, of which 

 the Cascalho often rests on gneiss, which when bare is found to 

 be pot-holed. Diamonds are often found in the holes. Mr. 

 Nicolay considers the Cascalho near the coast to be more silicious 

 than at the Chapada. 



Combining his opinions vnth Mr. Hartt's, we must conclude 

 that the diamond in Bahia does not belong to the Itacolumite, 

 but to the sandstones of the Chapada, which have lent their 

 detritus to the Cascalho. 



