MUSEUM OF COMPARATIVE ZOOLOGY. 163 



crushing, aud 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 in 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 pi'operties 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 

 crystals 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 solutioti 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 tlie scale of metamorphism, occurring, as it 

 does often, in or associated with rocks that retain at least a part of 

 their original chai'acters. Its late formation in the rock, posterior to 



