colours which accompany them in calcareous spar. 277 
crystal was bounded by the artificial faces afh d, e beg, and 
having polished these faces, I transmitted a pencil of light 
through the interrupting plane. In this case there was nei- 
ther a multiplication of images, nor a production of colour, 
and the same result was obtained though I caused the pencil 
to fall upon the interrupting stratum at the same angle at 
which it was incident when the images were multiplied and 
coloured. Hence, it is obvious, that if the colour's were 
produced by a fissure, they ought still to have appeared even 
when the fissure was bounded by parallel plates of spar. 
In the specimen which is shown in Fig. 7, we are presented 
with several curious facts relative to the interrupting plane. 
This specimen is intersected by three interrupting strata afhd, 
ebeg, ejQjey, the two first being equidistant from AB, and all of 
them having the same position relative to the axis of the 
rhomboid. The thickness of the interrupting strata is dis- 
tinctly seen at af and eb, and is nearly dth of an inch, 
bounded by two distinct parallel lines. The upper surface 
af of the stratum afhd is on a level with the general surface 
AEBF, but the upper surface eb of the other stratum forms 
an angle of 141 0 with the plane eE b and is smooth and well 
polished. The lower surface dh is partly level with the gene- 
ral surface, and partly inclined at an angle of 141 0 to the 
plane d Hh, and the surface gr is parallel to the inclined sur- 
face eb. The two strata afhd , ebeg have therefore a crystal- 
lized structure, and as they effervesce with nitric acid, we are 
entitled to consider them as flat rhomboidal veins of calcare- 
ous spar. 
The stratum ebeg, which is shown separately in Fig. 8, is 
divided into portions by four or five veins mn, op, some of 
