Geology and Mineralogy. ; 223 
atchepanrtieaa HY the presence of isolated masses of crystalline 
ble. 
limestone o 
a detaniitnation of the geological age of these various rocks, 
and even that of the relative ages of the different groups, is ren- 
dered difficult by the absence of ate and by the excessive 
dislocation of the beds by folding and faulting, faults being 
pela: numerous and giving a peculiar character to the moun- 
of the region, which Mpabeat & hase a moderate slope on_ 
one tide and a ‘precipice on the o 
The rocks above described eve! ee referred to the Tertiary 
and Secondary ages; but there are good reasons 3 considering 
them more ancient than the limestones of the Sio Francisco in 
which Prof. O. A. Derby found fossil corals which, indicate that 
these are much older than the Secondary and belong to the Paleo- 
ZOIC a 
Th ie hes ore modern rocks are represented by the pare iron 
conglomerate denominated canga formed on the surface 
fragments of the underlying rocks and which continues a form 
to-day, and by deposits of lignite of Ter fans y age as is proved by 
the sei plants and fishes contained in 
t of ex eaoie sa" interest, frail an agricultural point of 
view, is the niform presence of a notable ina of alkalies, 
particularly potash, in al the schietiie rocks examined, and the 
ime in the same rocks. The first fact “e dlains ~“ 
wonderful fertility of sis of the soils derived from the dec 
position of the schists, and the second indicates the siluee fertil 
izer for the more sterile soils. 
the precious stones fond in Minas, the deposits of topazes, 
situated near Ouro Preto, have been most studie pazes 
4 and the still rarer euclases are found 3 in their primitive formation 
angles of 30° to 50° to the eastward. The schists are the pre- 
dominant rocks and belong to the two divisions already described 
: _ of clay schists and greasy or unctuous schists. They contain 
_ _pyrophyllite and embedded octahedral crystals of iron Pio hav 
ing the ae of and resulting from the alteration of pyrite 
us topaz mines that have been opened lie ‘toh two 
parallel ne running W.S.W. In th 4 Viste mine which is 
a deep open cut, the ‘beds explor of are alibabis shales of several 
varieties conan the talc-like mineral already mentioned. 
These beds are inclined to the eastward at an angle of 40° to 60° 
and are covered by superficial deposits of sand and conglomerate. 
€ gems occur in an irregular fracture or vein filled w ith a so oapy 
clay or lithomarge and running about W.S.W., or perpendicular 
to the strike of the country rock. The vein divides into ae 
some of which sometimes accompany the nee and is ofte 
split up into pockets in which the topazes are o f greater size and 
more abundant. Rarely topazes are found without the lithomarge 
