THE GRAIN. 27 
Renard states that the scales of chlorite lie perpendicular to the cleavage, i. e. 
about in the direction of the grain. « Jannetaz's experiments in reproducing grain 
have already been referred to (p. 8),&and his experiments showing thai the direc- 
tion of the grain is that of the greatest elasticity are given on page 45. 
Daubree, in one of his experiments, produces cleavage in the direct ion of pressure 
and motion — which is the relation of grain to pressure, c It seems, therefore, that 
besides the cause assigned to grain by Sharpe there is the formation of exceedingly 
obscure vertical divisional planes in the direction of the pressure and the crystalliza- 
tion of secondary minerals along these planes.^ The variation of the strike of the 
grain from the direction of the cleavage dip at Rimogne (Ardennes) is from 1 to 20°. 
At Fumay, in the same region, that variation is 6°. « At Rimogne the strike of the 
grain bisects the acute angle formed by two sets of joints. In some of the Ardennes 
slates plates of hematite lie in the "grain" and indicate its direction../' Watrin 
states that as the longer axes of distorted octahedra of magnetite all lie in the direc- 
tion of the grain in some of the Ardennes slates, their combined magnetism gives a 
polarity to the slate in the direction of the grain and enables the quarrymen to ascer- 
tain its direction by the magnetic needle. A like arrangement of lenses and dis- 
torted cubes of pyrite characterizes the slate at Northfield, Vt. (seep. 91), and of 
folia of muscovite in certain slates near New Canton, Va, 
The same feature is occasionally seen in schists. Thus scales of biotite in a schist 
in Beekman and Pawling, Dutchess County, N. Y., and actinolite crystals in schist in 
Hubbardton, Vt., have their axes in any direction, but their flat sides transverse to 
bedding and cleavage. 
The "grain" shows itself in a more or less obscure striation of the cleavage surface 
in a direction nearly parallel to the cleavage dip and sometimes to the dip joints. 
In several of the Maine quarries the grain is abnormally nearly horizontal, while 
the bedding is steeply inclined. As the strike of the grain is about the same as the 
direction of the pressure which produced the cleavage the logical inference seems to 
be that the folds at these quarries, if reconstructed, would be found to have a nearly 
vertical pitch, and this would have to be attributed to a secondary movement. The 
workmen at these quarries speak of the slate as being "on end." The slate blocks 
are broken along the grain to reduce them to workable proportions, and as the grain 
is the direction of weakness roofing slates are always cut with their long sides parallel 
to the grain, but in some localities there is hardly any grain. 
In making thin sections for the microscopic studies of the writer for this bulletin 
fresh specimens were obtained from the quarries with the grain direction marked on 
them by the foremen. Thin sections were then prepared transverse to the cleavage 
and parallel to the grain and also at right angles to it. Where any difference is 
noticeable between the two sets it consists in the presence of large flakes of chlorite 
with their flat sides lying in the grain direction or in the longer axes of lenses and 
crystals lying also in that direction, and, where the sections are very thin indeed, in 
many small scales of muscovite being similarly oriented. Where the matrix is very 
micaceous and not obscured by carbonate or other minerals, sections parallel to the 
grain polarize more brilliantly than those across it. 
JOINTS. 
Nature of joints. — Joints are simply ruptures of continuity due to various strains. 
Exceptionally, later movements may cause slippage along joint planes and result in 
polishing the joint faces (slickensides). The usual character of joint planes, how- 
a Renard, op. cit., vol. 3, p. 235. 
*>See Jannetaz, Memoire sur les clivages des roches (schistosite\ longrain), et sur leur reproduction: 
Bull. Soc. Geol. France, 3d Ser., vol. 12, p. 211, 1883-84. 
c Daubree, Etudes svnthetiques, etc., p. 422. (For lull titles to works cited see Bibliography, p. 138. 1 
dRosenbusch figures biotite' scales transverse to cleavage: Elemente der Gesteinslehre, fig. 73 
(p. 432), p. 437. 
e Renard, op. cit., vol. 3, p. 3. 
/Daubree, op. cit., p. 336. 
