THE GREAT PALISADES OF THE MINNESOTA COAST. 31% 
which I myself failed to find. All I could find was a series of plainly- 
marked flows, with massive columnar lower portions and upper vesicular 
portions as strongly developed as anywhere in the typical region of Ke- 
weenaw Point. The red detrital material appears to me to always occupy 
the open spaces of the scoriaceous upper portions of the lava flows. In the 
thin section it is invariably sharply defined from the matrix, which always 
presents the usual appearance of the diabase-amygdaloids. ‘The whole 
occurrence is closely like that of the ashbed rocks of Keweenaw Point, 
both as to the curious intermingling of scoriaceous amygdaloid and detrital 
material, and as to the peculiar kind of diabase forming the lower portion 
of each flow.’ 
Above these beds comes the mass of quartz-porphyry which forms 
the Great Palisades. The entire thickness of this porphyry is over 300 
feet, but in the section under description only some 50 to 75 feet are in 
sight. The base of this mass of porphyry presents a most peculiar appear- 
ance. For a thickness of some 5 feet it is much weathered, and calcified— 
the contact line of the dissimilar rocks having evidently been the course of 
altering waters—and shows a strong appearance of contorted lamination, 
which is often intensified by the calcitic alteration. This peculiar alteration 
is the same as that which affects, in a less prominent degree, higher portions 
of the mass, as is very distinctly seen in the thin section. The feldspar 
crystals are found following the curving lines and again obstructing them. 
The quartzes are much smaller than the feldspars, and are in the usual 
doubly terminated crystals, with embayments of the matrix. There are 
also contained in the quartzes fine glass inclusions, in regular shapes cor- 
responding to those of the containing crystal, and affected by a hair-like 
devitrification. Figs. 11 and 12 of Plate XIII represent this laminated por- 
phyry, and Figs. 3 and 4 of Plate XII, the rock at the northeast end 
of the Palisades. All of these figures represent those portions of the 
rock which show the flowage structure most plainly. The columnar char- 
acter of this rock is very noticeable, and is of especial interest, since a 
1Jt should be repeated here that both Norwood and Winchell regard the amygdaloids, the diabases 
that go with ‘them, and all of the felsitic porphyries as altered sediment. Compare Eighth Annual 
Report of the Geological and Natural History Survey of Minnesota, p. 26. 
