Alfred Harker—“ Eyes” of Pyrites in Slate. 397 
the cleavage-planes and drawn out along the line of cleavage-dip 
(lonyrain of the French). It indicates that in those cases the 
pyrites is of very early origin, and that it became coated at an 
early stage with a chloritic envelope; further, that the rock became 
thoroughly compacted before it was subjected to the lateral thrust, 
so that in yielding it was able to leave vacant spaces, which were 
filled at a later date by crystalline quartz. This last point is proved 
by the manner in which the quartz has been formed with its 
crystals perpendicular to the faces of the pyrites. There are, how- 
ever, other cases in which the quartz has a fibrous structure, the 
fibres being set parallel to the longrain, that is, to the direction of 
movement, as diagrammatically shown in fig. 2. This is presum- 
Sections at right-angles to the cleavage-planes. P=pyrites; Q=quartz; 
C =chloritic mineral. 
ably due to the quartz having been deposited concurrently with the 
process of deformation, so that no vacant space was actually formed. 
The examples cited by Loretz' from the Thuringerwald must be 
referred to this kind of action. In these the pyrites has been sub- 
sequently converted into limonite. 
Another variety of ‘“‘eye” is found in the Llandeilo slates of 
Whitesand Bay near St. David’s. Here the whole external surface 
of the “eyes,” which are about an inch and a half to two inches 
long, has a strongly fibrous, slickensided aspect. On making a 
section, it is seen that the pyrites cubes have their angles and 
quoins rounded off, and little fragments of pyrites are detached and 
enveloped in the quartz which occupies the corners of the “eyes.” 
It should be noted that these lenticular or eye-shaped masses are 
essentially characteristic of discontinuous sliding movement, in which 
actual disruption has taken place between substances capable of 
different degrees of yielding, such as the hard pyrites and the less 
hard rock surrounding it. The same rocks often exhibit the ellip- 
soidal green spots (distorted spheres) first noticed by Dr. Sorby, 
which indicate continuous deformation without abrupt slipping. 
1 Jahrb. d. kénigl. preuss. geolog. Landes. for 1881, pp. 283-289. 
