o 
FELSITE AND QUARTZ-PORPHYRY. 101 
partially altered glass, I am uncertain. The other occurrence referred to is 
that of curvilinear aggregations of brown and red ferrite particles, large 
enough to be seen macroscopically, in the hand specimen, as hair-like 
markings. These characterize the felsite from the Minnesota coast above 
mentioned as remarkable for the coarseness of its secondary quartz network. 
This rock is illustrated in Figs. 15 and 16 of Plate XIII. 
The porphyritic feldspars in the thin section are found to be either or 
both of orthoclase and oligoclase. They are always turbid from decompo- 
sition, and are more commonly red-stained than not. They have always 
crystalline outlines, or, when they have been eaten into by the still fluid 
matrix, as is not seldom found to have been the case, at least the remnants 
of such outlines. In a number of sections the feldspars are seen to have 
been not only eaten, but also much shattered before the solidification of the 
surrounding magma. 
The porphyritic quartzes present all the usual characters of the quartzes 
of similar rocks the world over. They are random sections of dihexahe- 
dral crystals (double pyramids due to combination of the two rhombohe- 
drons), with now and then some development of the lateral (prismatic) faces. 
The rhombohedral angle being only a few degrees over 90° (94°.15), the 
sections of these crystals present a nearly square shape.’ Usually they 
are more or less rounded and eaten into by the matrix, many odd forms re- 
sulting from this corrosion. In nearly all cases, however, some traces of 
the original outline remain, with the aid of which, along with the behavior 
between the crossed nicols, it is always easy to ascertain the erystallo- 
graphic directions of these eaten crystals. In the series of figures on Plate 
XIII, I have placed a number of these quartzes with the crystallographic axes 
in a vertical position so that they may be compared with one another.” 
Included in the porphyritic quartzes are particles or patches of the red- 
stained microfelsitic or eryptocrystalline groundmass. In most cases these 
have had originally a connection with the rest of the groundmass by 
‘N. H. Winchell (Ninth Annual Report of the Geological and Natural History Survey of Minne- 
sota, pp. 21, 33, &c.), has called the dihexahedral quartzes of the quartziferous porphyry of the 
Palisades and other points on the Minnesota coast, adularia. 
*Rutley (Study of Rocks, p. 210), speaks of roundish blebs of quartz as characterizing quartz- 
porphyries generally, but his ‘‘ blebs” are only rounded crystals. 
