6 GEOLOGICAL MEMOIRS. 
pared to the forms we see on marbled paper or speckled woods*.: 
Lastly, the very remarkable condition of texture, that stretching’ 
of the gneiss and gneiss-granite, demands our closest attention, a 
phenomenon, which, however often it has been observed, has not 
hitherto met with due consideration. 
In my ‘Hints towards the establishment of a doctrine of the na- 
ture of rocks+,’ I long ago pomted out the fact, to which I have 
repeatedly since called attention, that the parallel texture of rocks 
must be understood in a twofold sense, and that they are very dif- 
ferent, the one the plane parallel texture or flattenmg (lamimation or 
foliation), and the other a linear parallel texture or stretching (ten- 
sion). I also endeavoured to show m a subsequent memoir{, that in 
the crystalline siliceous rocks the lamination is the result of pressure ; 
whereas the strike is to be explained by a drawing-out or protrusion 
of the mass; an explanation adopted. by all geologists with respect to 
the flattened and elongated vesicular cavities in lavas and amygdaloids, 
and which appears no less applicable to many kinds of long foliated 
gneiss and granular foliaceous gneiss-granite. 
In the well-known memoir of Sedgwick on the structure of large 
mineral masses $, reference is made to this appearance, which he calls 
the grain; and under that term it has in later times been frequently 
brought into notice. Fournet, in particular, in his beautiful memoir 
on the Alps between the Vallais and Oisans||, enters very fully mto — 
the question of the mode of formation of the planes of lamimation 
and of lmear parallel structure in gneiss and granite. ‘ When a 
viscid molten mass,”’ he says, *‘ free from all external influence, ery- 
stallizes, a granitic structure is produced; but if it be acted upon by 
certain forces, as, for example, by the lateral pressure of the wall of 
a fissure, in that part of the mass which is in contact with the wall, 
there will be a regular separation of the constituent parts ; and this 
process may be so often repeated, that at last the whole mass may 
consist of a succession of alternating beds.”” Farther on he proceeds 
to say, “An eruptive mass, by being forced through a more or less 
narrow fissure, may undergo an extension or flattenig, by which its 
different constituent parts will be squeezed flat and drawn out length- 
ways, producing a rock having a striped or riband structure, even a 
true gneiss. It is therefore quite conceivable that gneiss and granite 
may have one common origin, and it will often be very difficult to 
recognise in them two distinctly different kinds of rock.” 
But the most important observations and consequences dedueed 
from them, with which I am acquainted, are indeed those communicated 
* As, for example, frequently seen in the granite-gueiss of Norway.—See my 
‘ Beitrage zur Kenntniss Norwegens,’ Bd. ii. s. 166, and Scheerer im Neuen Jahrb. 
1843, s. 632, 638, u. a.o. 
+ Andeutungen zu einer Gesteins-Lehre, Leipzig, 1824, s. 57. 
+ Karsten und v. Dechen’s Archiv, Bd. xii., 1838, s. 23. 
§ Trans. of Geol. Soc. 2nd Ser. iti. 461. 
|| Ann. des Sciences Physiques et Naturelles publi¢es par la Soc. Roy. d’Agri- 
culture de Lyon, t. iv. p. 105. 
