Parr VIL. §3.] REGIONAL METAMORPHISM. 587 
tary matter we encounter alternations of different kinds of sediment, 
which could not but produce distinct kinds of rock under the influence 
of metamorphic change. In the second place, cleavage depends for 
its perfection and continuity on the fineness of grain of the rock 
through which it runs. While exceedingly perfect in a mass of 
argillaceous strata, it becomes feebler or even dies out in a coarse 
sandy or gritty rock. Hence, where foliation coincides with cleavage 
over large tracts, there will almost certainly be bands, more or less 
- distinct, coincident with the original stratification, and running 
oblique to the general foliation, like bedding and cleavage, save 
where these two kinds of structure may happen to coalesce. 
In a region of intense metamorphism the foliation of the schists 
becomes here and there somewhat indefinite, until, disappearing 
altogether, it gives place to a thoroughly granitic character. Between 
gneiss and granite there isno difference in mineralogical composition ; 
in the one rock the minerals are arranged in folia, in the other they 
have no definite arrangement. Gneiss might be called a foliated 
eranite; granite might be termed a non-foliated gneiss, and, 
indeed, the two rocks may sometimes be observed to graduate into 
each other. It has been naturally concluded that such granite is the 
ultimate stage of metamorphism. 
There is thus nothing improbable in the idea that the same 
mineral particles may have gone through many successive cycles of 
change. We may suppose them to have been originally part of a 
granite mass, and to have been subsequently exposed at the surface 
by enormous denudation. Worn away from their parent granite they 
would be washed down with other particles, and spread out under 
water as parts of sandy or muddy deposits. Buried under a gradual 
accumulation of sedimentary material thousands of feet in thickness, 
they might be depressed deep beneath the surface, and be thus 
brought within the influence of metamorphism. Gradually recom- 
posed, crystallized, and converted into schistose rock, they might be 
eventually reduced to a soft or pasty condition and protruded into 
some of the overlying less metamorphosed masses in the form of 
granite veins. Or we may conceive, that a communication was 
opened between the granite thus produced and the surface, and that 
the original mineral particles, whose vicissitudes we have been tracing, 
were finally erupted to the surface as part of a stream of lava (p. 545). 
Possible Metamorphism of Igneous Rocks.—In most 
large tracts of foliated rocks there occur masses less distinctly 
foliated or quite granitoid in texture, formed mainly of hornblende 
or of that mineral in combination with others. Zones or bosses of 
hornblende-rock and hornblende-schist frequently appear among 
gneiss and mica-schist. Varieties of quartz-porphyry occur in a 
similar way. Bands of fine unctuous chloritic or hydro-mica schists 
may also often be traced. It is not easy to understand how such 
rocks, at least those containing a large percentage of magnesia, 
could be produced by the metamorphism of ordinary sediment, 
