Reviews—A. Rénard—On the Ardennes. B28 
tion of certain anomalies, as, for instance, where the alumina in the 
sericite may be largely replaced by ferric oxide, and also where the 
same mineral may contain a portion of the magnesia assigned to 
the chloritoid. ‘Traces of phosphoric acid, sulphuric acid, lime, and 
organic substance may be regarded as usually present. 
Thirteen analyses are given of various examples of “ phyllades,” 
including one of the manganese garnet rock (ceticule) of Viel Salm, 
and the proportions of sericite, chloritoid, and free quartz are 
deduced from the per-centages found and tabulated in the ordinary 
way. The calculated amounts of these minerals are then exhibited 
in a second table for each analysis, and some brief microscopic notes 
are given for each specimen. In practice the sericite is credited 
with all the alkalies, and the corresponding atomic weights of silica, 
alumina, and water allotted to that mineral. In the same way the 
chloritoid is credited with all the magnesia and the ferrous oxide, 
and receives the rest of the alumina and of the water together with 
its proportion of silica, the balance of which is estimated as quartz. 
Sericite, as before noted, is the most constant in quantity, and also 
the most abundant. It ranges from 56 to 23 per cent. Chloritoid, 
including, in one case, the variety ottrelite, ranges from 46 to about 
10 per cent., and, roughly speaking, is about half as plentiful as 
sericite. The quartz ranges from 45 to 13 per cent. 
Reverting to the analyses themselves, we may note that the Silica 
in these phyllades ranges from 65 to 45 per cent. It is highest in 
the green zones, and lowest in a pale grey phyllade in contact with 
a kind of dicrite. 
Titanic acid is a constant accompaniment, mostly ranging from 
$ to 14 per cent. It would seem to occur as rutile rather than as an 
ingredient of ilmenite. 
Alumina is abundant, attaining to nearly one-third of the whole 
in the more basic specimens, but usually from 20 to 25 per cent. In 
most cases the Ferric oxide only amounts to 2 or 3 per cent., but in 
one case largely replaces alumina. 
Magnetite—Some specimens contain 38 or 4 per cent. 
Ferrous oxide is not very abundant, ranging from less than 1 per 
cent. to about 8 per cent., though in one case it reaches as high as 
54 per cent. Lime is very scarce, even in the basic specimen in 
contact with the diorite. The per-centage of magnesia is pretty 
steady at between 1 and 2 per cent. | 
The alkalies are also pretty constant in their amounts, potash 
being usually three and a half times more plentiful than soda. The 
general average of the two combined is from 4 to 5 per cent. In 
the most basic specimen the alkalies amount to over 6 per cent., 
potash being very nearly four times the amount of soda. 
The water ranges from about 3 to 4 per cent. A synoptical table 
at p. 84 gives the aggregate results; and we are further promised a 
description, in the second part of the work, of the physical characters 
of each of the elements of the “‘ phyllades”’ of the Ardennes. 
The following extract from the work of Dr. Barrois, reviewed at 
p. 273, may serve to give some idea of what is meant by this term. 
