18 Anniversary Address. 



Joao do Barro, wliere, in 1850, a chance wasTiing of tlie under- 

 lying schist disclosed the presence of many diamonds. The 

 deposit is not confined to the beds of rivers or ravines, but covers 

 the slopes and tops of the hills. This deposit ceases exactly at 

 the boundary of the bituminous beds of the coal measures of St. 

 Catherine. 



Is this, then, I would ask, any indication of the origin of 

 diamond in Carboniferous rocks ? If so, ought not those rocks 

 to contain diamond ? 



In the north part of Minas Geraes, Jurassic calcareous form- 

 ations cover the red sandstone, and these are in turn subordina^te 

 to gypseous marls and rock salt. Yet in the ravines, cut 

 down to the sandstone, through the overlying beds, diamonds are 

 found, i.e., above the Carboniferous formation. 



Moreover, in 1839, diamonds were discovered in the Psammite 

 of Serro de Santo- Antonio de G-rammagoa imbedded in the rock, 

 whereas in the preceding Itacolumite sandstone they occur be- 

 tween the plates of mica, just as garnets occur in mica-schist. 

 The edges of these are rounded, whilst in the Psammite the 

 angles are uninjured, proving that the transmutation of the 

 sandstone into Itacolumite has also affected the diamond crystal- 

 lisation. Por a long period the red Psammite and the secondary 

 Itacolumite sandstone have been regarded as the sole matrices of 

 diamond, whence it has been derived by the detrital erratic de- 

 posits ; but since then, it has been found in the true Itacolumite 

 subordinate to the talc schists with quartz. 



The diamonds in the derived deposits are more numerous the 

 nearer the deposits are to the solid rocks. 



The detrital beds are classified according to their character. 



Thus Groupiara is a drift not due to the present system of 

 drainages. Burgallio or Gurgallioa consists of superficial frag- 

 ments of underlying rock. The decomposed schist of the latter 

 is called Barro. A sandy mass between these is spoken of 

 as Tei-ra. Another bed of granular Itacolumite is known as 

 Fizarro ; but all belong to the decomposed rocks. 



Cascalhao represents the sand, clay, and pebbles in the beds of 

 rivers, torrents, lakes, and of the hollows in their courses through 

 the solid rocks. Talioa-canga or Tapahan-canga is what we call 

 in Australia cement. 



