108 Mr. Dana on the Analogies between the 
If these principles are applied to granitic or ancient Plutonic 
rocks, including the associated schists, we may explain all their 
peculiarities of structure without other aid. In common granite 
the feldspar predominates much over the mica, and fixes the di- 
rection of its cleavage planes. Gneiss which contains more mica, 
has both the cleavage of mica and feldspar, the former at right 
angles with the latter. ‘The mica was so abundant that the form- 
ing crystals felt that mutual influence, which causes them to take 
_ parallel or homologous positions, and so by arranging itself in 
planes, gave rise to that appearance of stratification which distin- 
guishes gneiss from granite. In mica slate, the feldspar is whol- 
ly subordinate to the mica, and the structure is very distinctly 
foliated, almost like mica itself. A very little mica with quartz 
in grains, produces a rock with a micaceous structure, because 
quartz has no cleavage of its own. 
Hornblende rocks, from syenite to hornblende slate, form a par- 
allel series to the above, explained on the same principles. But 
as hornblende is less easily cleavable than mica, so hornblende 
slate is just so much less cleavable than mica slate. 
We hence conclude, and not without reason, that the schistose 
structure of these rocks results from their constitution, and that a 
fine-grained granite with the amount of mica in mica slate, could 
no more exist without a foliated structure, than mica itself could 
crystallize in blocks like feldspar. 
We may derive another argument on this subject from the 
metamorphic theory itself, which supposes that gneiss and mica 
slate were once beds of clay or argillaceous sandstone. Judging 
from the nature of such deposits—say, for example, those of the 
carbonaceous era—we should never believe that the elements of 
mica contained in them, lie in alternating layers, and in so thin 
alternations as mica presents in micaceous rocks. It is far more 
probable, if the rock be considered an altered clay, that the mica, 
when crystallizing, sought out its own positions upon the crystal- 
lographic principles already explained, and the same result would — 
take place, and a rock equally foliaceous be formed, whether the 
beds of clay were stratified or compact. 
Not to delay longer on this branch of the subject, I proceed 
with my second proposition, that some granites may have had a 
metamorphic origin. 
