9o8 The Ajnerican Naturalist. [October, 
cally improbable. Further, each belongs to a different type, and there- 
fore would not be expected to be pseudomorphic. Clarke suggests that 
all of the true micas are isomophous combinations of derivatives 
of aluminium orthosilicate [Al/SiOJj] and of the corresponding 
polysilicic compound [Al,(Si30g)3]. The different micas are taken up 
seriatim, and it is shown how each may be regarded as a compound pro- 
duced by the isomorphous mixture of derivatives of these. Muscovite, 
for instance, may be looked upon as Al3(SiOj3KHj, in which one 
atom of Al has been replaced by potassium and hydrogen, and lepidolite 
as a mixture of AljCSiOJjKHLi and Al(Si30g)3K3Li3(AlFj3. The 
members of the clintonite group are also regarded as related to 
the micas, and to have the general formula R'j^SiO,— Al=R"Oj. 
The formulas suggested by Prof Clarke certainly have the advantage 
over those of Tschermak as far as simplicity is concerned, but whether 
they will prove of more value in the discussion of the composition of 
the micas, it remains for further work to determine. Among a few 
minerals 7 recently described from Brazil the following present some 
points of interest. Zircon from the sands of Rio Verdinho, near 
Caldas, Minas Geraes, shows on cleavages parallel to oo P two systems 
of striations, one parallel to the diagonal of the cross sections, and 
another nearly norrnal to the P face. Sections perpendicular to the opti-# 
cal axis are uniaxial in some parts, and in others show a biaxial figure, 
probably due to a very fine system of twinning lamellae. Fine large 
apatite crystals from the red orthoclase of a coarse-grained gneiss from 
Pedreira de Sandade, near Rio Janeiro, resemble very closely the 
apatites of Renfrew, Canada. They are of various colors, and possess 
the rounded contours of the Canadian specimens. Grten fibrolite from 
Diamantia owes its color to the inclusion of numerous needles of 
tourmaline. Miers and Prior ^ have recently made a very thorough 
examination of proustite and pyrargyrite. Their article begins with 
an historical and critical review of the work previously done on these 
interesting minerals. It continues with a discussion of the morpholo- 
gical and physical properties of all the crystals in the possession of the 
British Museum, and a review of the work of former mineralogists. 
The paper concludes with the results of analyses of ten specimens of 
pyrargyrite and five of proustite, and is accompanied by a plate 
on which are represented twenty-eight figures of crystals. Their 
results may be briefly summed up as follows: (i), proustite and 
pyrargyrite are distinct species, the former with a rhombohedral 
•< Dom Pedro Augusto. Min. und. Pet. Mittk., X., p. 451. 
8 Zeits. fur Kryst., XV., p. 129, and XIV., p. 113. 
