ee ee ee 
Papa eee ee ee ee 
NAUMANN ON GNEISS AND GNEISS-GRANITE. 7 
to us by Darwin in his two works, ‘ Geological Observations on Vol- 
canic Islands,’ and ‘ Geological Observations on South America.’ He 
saw in the island of Ascension a volcanic rock, composed of felspar, 
diopside and quartz, having a perfect gneiss-like texture and struc- 
ture, the alternating layers of the component parts being extremely 
fine, and extending parallel to the direction of the lava-stream. The 
explanation he gives of this is a very correct one; viz. that flowing 
slowly downwards in a viscid state, the mass was subjected to an in- 
ternal stretching of all its constituent parts, while at the same time it 
was subject to pressure from the mere force of gravity; and he refers 
to Forbes’s description and explanation of the parallel structure of 
glacier ice*. 
Darwin informs us, that in the Cordillera of Chili, great beds of 
a.red granite occur, which must be viewed as an eruptive rock ; but 
that it nevertheless exhibits, in parts, a decidedly parallel structure. 
In the gneiss of Bahia he observed included angular masses of 
a hornblende rock, which are indubitably fragments. The gneiss 
in the neighbourhood of Rio Janeiro has a porphyritic structure 
with imbedded crystals of felspar three or four inches long; and 
although there be no parallel alternation of the constituent parts, still 
it has a parallel structure or grain in the mass, and in some places 
it does alternate with true gneiss beds. Darwin on this occasion says 
expressly that the parallel structure, and even the foliations, afford 
in his opinion no valid objection to this gneiss-granite bemg con- 
sidered rather as an eruptive rock than as a metamorphic formation. 
In Botofogo Bay, not far from Rio, a colossal fragment with sharp 
edges of another variety of gneiss, containing much mica, is found 
imbedded in the same gneiss-granite. In a subsequent passage he 
mentions the great dyke of gneiss in the mica-slate of Venezuela, 
formerly described by Humboldt, and in reference to the theory of 
its formation comes to the conclusion, that the parallel structure of 
crystalline siliceous rocks must very frequently have been modified 
by the tension to which they had been subjected throughout the whole 
area of eruption, before their final consolidation +. 
When a geologist of so high authority as Darwin announces 
such an opinion on the formation of gneiss-granite, it may seem 
almost superfluous for me to refer to the instances of fragments of 
grauwacke-slate pointed out by myself as occurring in the gneiss of 
the Striegis valley, close by the place where it abuts against the 
grauwacke formation ;—to the great masses of clay-slate in the gneiss 
of the Castle Hill of Frankenberg, as also to the grauwacke fragments 
observed by Cotta in the gneiss of the Goldberg near Goldkronach ;— 
or to mention that Hoffmann had already called our attention to the 
remarkable conditions of the beds of gneiss in the Munchberg, obser- 
vations afterwards fully confirmed by what I myself saw in that loca-~ 
lity, which can in no way permit us to consider that formation as meta- 
* The interesting comparison by Forbes between the structure of glaciers and 
lava-streams will be found in the Edin. New Phil. Journal, vol. xxxvii. 1844, 
p. 231. 
+ Compare, in this view, my memoir in Karsten’s Archiv, Bd. xii. 1838, s. 23. 
