I50 The American Geologist. September, looi. 
planes have cut across twinning lamellae it was observed that 
no displacement of these had taken place and hence there could 
have been no shearing or slipping. The minerals traversed by 
the planes show no evidence of having been shattered. The 
alteration seems, in all cases, to have taken place along certain 
planes of stress produced by dynamic forces. Along these 
planes the rock has been subjected to a strain which 
has resulted, not in the mechanical deformation of the constitu- 
ents, but in the change of their mineralogical character. 
In the same rock the hypersthene is sometimes surrounded 
by a double rim of hornblende, the inner portion colorless, the 
outer green and more compact. 
Another metamorphic change in the noryte is the alteration 
of the feldspar into saussurite. In the study of these saussuri- 
tized rocks it is usually impossible to determine whether the 
original type was a gabbro or noryte, since all the pyroxene has 
been transformed into hornblende. In a specimen collected on 
the west side of Octoraro creek just south of the "horseshoe 
bend" remnants of the hypersthene still remained in the fibrous 
hornblende, leaving no doubt that in this instance at least the 
rock was a noryte. The orthorhombic pyroxene had lost much 
of its color, but was yet slightly pleochroic and contained the 
characteristic tabular inclusions. The feldspar had been en- 
tirely replaced by a whitish or greenish, opaque substance com- 
posed almost wholly of zoisite. In thin sections this mineral 
was colorless, with a rather high index of refraction but low 
double refraction, the interference colors being sometimes dull 
gray, more often a beautiful ultramarine blue. 
While it is not an uncommon thing for the gabbros to have 
their feldspar thus altered to saussurite, this is, so far as known, 
the first instance reported of a like alteration in the norytes. 
We have here a true saussurite noryte composed of zoisite and 
partially uralitized hypersthene. It is not improbable that many 
of the rocks described later as saussurite gabbros are really 
norytes. 
Five varieties of the noryte occur within the area under con- 
sideration, namely : 
1. Noryte. 
2. Porphyritic noryte. 
3. Ouartz-noryte. 
