GRANITE -GNEISS IN CONNECTICOT 643 
thin-bedded gneiss and schist which contain both biotite and horn- 
blende, and with amphibolite. These dark bands of gneiss and 
schist are not here variable in direction, but run parallel to the 
boundary of the granite-gneiss, and agree in dip and strike with 
the schist east of itand with themselves. Yet undoubted igneous 
contacts occur between these dark gneisses and schists, and the 
lighter granite-gneiss which occurs between them. The granite- 
gneiss which in other parts of the area replaced almost entirely 
the schist through which it came, here merely forced its way 
into fissures parallel to the foliation of the country rock. The 
distribution of this belt of mixed rock is shown on the map. 
As already pointed out the granite-gneiss gradually disappears 
as we go north. 
4. Schheren.—-\n a number of places where the granite-gneiss 
is most massive, it encloses patches of darker material or 
schlieren—basic segregations of the granitemagma. These are 
best seen in the Benvenue and Maromas quarries. The schlieren 
are of uniform composition, finer grained than the enclosing rock, 
have a larger proportion of biotite, and often contain hornblende, 
which is almost wholly wanting in the ordinary granite-gneiss. 
They are rounded, elliptical or irregular in outline and always 
elongated parallel to the foliation so as to form lenticular patches 
and in extreme cases bands. This lenticular and frequently 
sheet-like form is probably due to movement in the partly 
differentiated magma previous to thes olidification of the rock. 
Such darker and more basic portions are very common in gran- 
ites and have been regarded as a strong indication of the eruptive 
origin of the rock in'which they are found. In the Benvenue 
quarry where these schlieren occur,:the rock also holds irregular 
inclusions of schist. 
Associated Pegmatite Dikes—Although the presence of abun- 
dant pegmatite dikes~about a granitic area would not in itself 
be a proof that the rock in question was eruptive, it would 
be an interesting and corroborative fact. In the opinion of 
* ROSENBUSCH : Massige Gesteine, p.62; G. H. WILLIAMS, XV., Ann. Rept. U.S. 
Geol. Surv., p. 662. 
