FELSITE AND QUARTZ-PORPHYRY. 97 
various shades mentioned in my notes being ‘“‘brick-red,” ‘dark red,” “dark 
purplish-red,” “dark brownish-red,” “brick-red, banded with lighter shades,” 
“dark-purple, banded with indefinite markings of light-red,” ‘dark purp- 
lish-gray, blotched and banded with brick-red,” “pale-lilac,” “‘pinkish-lilae,” 
“pink” and ‘“‘light-gray.” Often there is no appearance of banding, but as 
often one produced by waving or contorted lines or rows of spots of lighter 
material. Occasionally this banding becomes very pronounced, as in the 
lower part of the flow at the Great Palisades on the Minnesota coast, 
where it is plainly a flowage result, even as seen macroscopically. In one 
case, that of a pink felsite on the Minnesota coast, in the SW. 4, Sec 28, T 
56, R. 7 W., the flowage structure is brought out on a grand scale by 
curving bands and §-like forms, sometimes several inches in width, of a 
darker, more highly ferritic felsite. This rock is fully described and pictured 
in a subsequent chapter. 
The matrix is always aphanitic, and has often a pronounced con- 
choidal fracture, but this is never quite so prominent as in the diabase- 
porphyrites above described, and is at times entirely wanting, the fracture 
surface presenting a rough and even hackly appearance. These felsites fre- 
quently come out of the ledge in sharp-edged, angular and even tabular 
fragments, while a very common result of weathering is the leaving of wedge- 
shaped or three-cornered cores, which fall from a cliff-side in showers at 
the slightest blow of the hammer. ‘The silica content of the aphanitic matrix 
appears to be high, ranging in all cases where a determination was made 
between 72 and 77 per cent. The matrix is fusible before the blow-pipe, 
but with difficulty. 
Of the porphyritic ingredients the feldspars are the most commonly 
present, appearing macroscopically in regularly outlined crystals, which are 
usually of a red color, though occasionally white and porcellaneous, and 
range in size from particles so minute as to be barely visible to the naked 
eye, to crystals a fourth of an inch and even half an inch in length. The 
porphyritic feldspars often show striated surfaces. In the banded porphy- 
ries they sometimes conform roughly to the contorted banding, forming 
strings of crystals ; and again they lie across the course of the bands, which 
will then curve around them. The whole structure is plainly one due to 
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