MUSEUM OF COMPARATIVE ZOOLOGY. 163 
crushing, and at the edges have yielded small broken quartz, which is 
mingled with the muscovite of the cement. The latter is made up of 
fragments of detrital quartz, quartz derived from the crushing of the 
large fragments, and perhaps some quartz formed chemically 2 situ, 
with muscovite filling the interstices, and even filling the cracks made 
in the large grains, and therefore evidently of metamorphic origin. 
There are also larger fragments of quartzite, and rounded aggregates 
of quartz and muscovite, which represent decomposed clastic feldspar 
grains. 
The ottrelite occurs in this cement in plates of irregular shape, often 
moulding itself around or enclosing the grains of quartz. It has all the 
optical properties of the ottrelite described above, and also encloses the 
quartz grains of the cement, but not the muscovite, and very rarely 
exhibits the least bending or straining ; hence it must have formed after 
the crushing forces had ceased to act. There are sometimes skeleton 
erystals of ottrelite, the hollow having the shape of an hour-glass, and 
transitions to crystals in which the hollow is filled up :by ottrelite 
enclosing quartz. The cement also contains the black metallic plates, 
small and imperfect, which are sometimes enclosed in the ottrelite. 
We may conclude from the microscopic study of these rocks that the 
ottrelite was the last mineral to form in them: it encloses the grains of 
quartz of the cement, both when they are easily, recognized ‘as clastic 
in the grauwacke and when of doubtful origin in the fine-grained schist. 
The muscovite, which is evidently a metamorphic mineral in both 
rocks, formed before the ottrelite, although not enclosed in it, for in 
position and arrangement it is not affected by the latter, and it seems 
necessary to suppose a chemical solution of the muscovite which filled 
the space between the quartz grains at the time the ottrelite came to 
fill that space. The ilmenite-chlorite plates also formed before the 
ottrelite, since they are enclosed in §it. 
In the grauwacke the muscovite is found penetrating the crushed 
pebbles of quartz along the cracks, and even penetrating into the sub- 
stance of the quartz a minute distance where there is no visible break, 
indicating a marked mobility for the solution from which this mineral 
formed. The ottrelite, on the other hand, forms in comparatively large 
unbroken areas enclosing the other minerals, somewhat analogous to a 
concretionary formation. Such an ottrelite grauwacke illustrates anew 
the position of ottrelite in the scale of metamorphism, occurring, as it 
does often, in or associated with rocks that retain at least a part of 
their original characters. Its late formation in the rock, posterior to 
