Dec. 7, 1882 | 
NAGOGRE 
123 
Coblentzian division of the Lower Devonian rocks of that 
region, that they pass insensibly into ordinary sedimentary 
rocks, but towards their axis have been metamorphosed 
into more or less crystalline compounds in which various 
silicates (garnet, hornblende, mica, &c.) have been deve- 
loped. He observed fossil ,lants and animals in some 
parts of these altered rocks. In cne of his specimens of 
a rock full of garnet, Sandberger determined the presence 
of the characteristic Devonian shells, Spz7zfer macropterts 
and Chonetes sarcinulatus. Nothing can be more em- 
phatic than the testimony borne by Dumont to the age of 
these rocks and the fact of their metamorphism. His 
essay upon them is hardly known to geologists generally, 
but it deserves to rank as one of the most precise and 
detailed contributions ever made by a field-geologist to 
the study of the phenomena of metamorphism,’ His 
observations have been singularly confirmed_by those of 
M. Renard. The metamorphic phenomena of the Ar- 
dennes are repeated on a greater scale in the extension 
of the Devonian rocks eastward into the basin of the 
Rhine, where they have been admirably described by 
Lossen,? whose pregnant memoirs on this and other geo- 
logical problems deserve the closest study of the student. 
Bringing all the assistance which chemical analysis and 
microscopical investigation now supply to the study of 
the origin of rocks, M. Renard, in the present communi- 
cation, which fitly opens tke first number of the newly- 
organised Bulletin au Musée Royal de Belgique, presents 
us with a detailed description of the garnetiferous and 
hornblendic rocks of Bastogne in the south-eastern por- 
tion of the Belgian Ardennes. It is impossible to give 
any adequate 7éswmé of this memoir within the space 
here available. But attention may be directed to one or 
two of its more interesting features. 
At the outset it should be noted that the band of 
metamorphosed strata here referred to occurs along a 
line of plication running in a general east-north-east and 
west-south-west direction; that it is not associated with 
any visible eruptive rocks, that it dies away into ordinary 
unaltered greywacke and shale on the outside, and be- 
comes more and more crystalline towards the axis, until 
it presents the most intense metamorphism anywhere to 
be found in Belgium. 
In subjecting to microscopic examination thin slices of 
some of these altered rocks, M. Renard noticed that the 
quartz-granules, presumably of clastic origin, have lost 
the liquid inclusions so generally found in the quartz- 
granules of old sedimentary strata. This fact (already 
observed by Sorby in the case of sandstone invaded by 
dolerite) seems to indicate that the sand-grains have not 
escaped the influence of the changes which have so pro- 
foundly affected ihe other constituents of the rormer 
sediment. ‘The original carbonaceous matter of the 
rocks, now altered into graphite, is spread as a fine dust 
among the other constituents, generally coating the 
minerals, scmetimes inclosed within them, frequently 
accumulated at certain points into black, brilliant irre- 
gular bands, occasionally as hexagonal flakes. This 
aggregation of the carbon recalls the way in which the 
graphite occurs in Archean limestones. The garnet 
Xt «« Memoire sur les terrains Ardennais et Rhénan.’’? Mem. Acad. Roy. 
de Belgique, 1848. 
2 “*Geognostische Beschreibung des Taunus,” &e. 
Deutsch, Geor, Gesell., 1867, p. 509- 
Zeitschrift der 
crystals are marked by a singularly interesting arrange- 
ment of lines of crystalline inclusions disposed along the 
crystallographic axes of the inclosing crystals. In certain 
rocks the garnets (about three millimetres in diameter) 
are traversed bya series of paralleled joints or fissures 
which run in a given direction through all the crystals. 
These cannot, of course, be cleavage lines. They are 
attributed by M. Renard to fracture produced by mecha- 
nical pressure, and he remarks that the minute flakes 
interspersed through the ground-mass of the rock are 
oriented in the same direction. 
Taking a general view of the microscopic structure of 
these rocks the author divides the constituent minerals 
into two groups: those which represent more or less 
distinctly the original sediment of which the rocks 
were formed, and those which have been subsequently 
developed by metamorphism. The quartz grains, for 
example, have preserved the closest resemblance to 
those of the ordinary normal arenaceous rocks of the 
lower Devonian series. The presence of graphite and 
anthracite likewise connects these crystalline masses with 
the sandy strata containing diffused carbonised vegetable 
matter. But on the other hand the crystalline structure 
and the presence of such minerals as garnet, hornblende, 
mica, titanite, and others connects these undoubtedly 
Devonian rocks with the crystalline schists of the Archzean 
series, as possibly both referable to the like series of 
physical and chemical changes. 
M. Renard unhesitatingly discards the doctrine of 
direct chemical precipitation. He admits that the evi- 
dence of the physical structure of the country, as Dumont 
so well enforced, demonstrates that these crystalline rocks 
lie in the Devonian system and pass laterally into ordinary 
sedimentary accumulations. He further insists that the 
study of the minute structure of the rocks under the 
microscope confirms, in the most satisfactory manner, 
the view of that geologist that the actual condition of the 
masses has been produced by metamorphic action, in 
what way soever this action may have been induced. He 
connects the metamorphism with the proofs of great 
plication traceable through the altered Devonian rocks of 
the Ardennes. The mechanical action involved in the 
process would, he believes, predispose the sedimentary 
materials to a more or less complete recrystallisation. 
As it crushed them under the enormous pressure and 
partly was itself transformed into heat, it would set into 
active motion the chemical affinities of the various 
mineral substances. In this way sand might finally pass 
into quartzite, argillaceous mud into phyllade or phyllite- 
schist, sandy clay into more or less schistose micaceous 
quartzites ; the calcareous matter would enter into com- 
bination to form the various lime-silicates so characteristic 
of these garnetiferous and hornblendic rocks ; while the 
carbonaceous ingredient, losing some of its constituent 
elements, would separate out as graphite. 
M. Renard’s testimony to the theory of metamorphism 
is all the more valuable, as it has been extorted from him 
by the irresistible logic of facts against his own previous 
convictions. He has now furnished to this theory fresh 
€vidence in its support, showing how well the observa- 
tions by which it is established in the field are sustained 
by minute petrographical analysis. Every one intere sted 
in geological research will hope that the paper he has 
