318 COPPER-BEARING ROCKS OF LAKE SUPERIOR. 
columnar structure is far less common in rhyolitic rocks than in basaltic.’ 
The columns are peculiar. The intersecting joints do not lie at the same 
angle with the vertical, but incline slightly toward each other, so that they 
intersect in depth. This is especially well seen on the south side of the 
Palisades, and is roughly indicated on the accompanying figure. 
Of all of the quartz-porphyries of the Lake Superior basin the Pali- 
sade rock presents the strongest appearance of sedimentary origin, on ac- 
count of the lamination it shows, especially in its lower portions. This 
lamination presents great irregularities, but in places, while changing many 
degrees in inclination within a few feet, it preserves for quite long distances 
the same general direction. Nevertheless, as shown in a previous chapter, 
this rock, like all the other porphyries of the Lake Superior basin, is of 
eruptive origin, and the lamination is that which often characterizes felsitic 
porphyries and rhyolites the world over. 
A mile and a half above its mouth, in the S. E. 4, Sec. 10, T. 56, R. 7 
W., Baptism River makes falls over a quartziferous-porphyry much like 
that of the Palisades. rom its position it appears not impossible that this 
rock belongs to the same belt with that of the Palisades. 
Below the Palisades for about a mile the coast is formed of a coarse, 
black, very highly olivinitie gabbro, with strongly marked vertically 
columnar structure. The diallages are large, producing a nodular weath- 
ering, and often show a shining metallic cleavage surface. The vertically 
columnar structure of this rock renders it evident that we have here to do 
with a flow and not a dike, from which it follows that there must be a fault 
between this rock and that of the Palisades, it being so different from the 
beds which belong beneath the Palisade porphyry. Beyond this rock again, 
and separated from it by a short beach, comes in a fine-grained, dark-brown 
ashbed-diabase with highly vesicular amygdaloids, the vesicles elongated 
in a common direction. After a short beach of only 20 paces, this is re- 
placed on the shore by a purple felsite, behind which it passes, forming the 
bed-rock of Baptism River a short distance above its mouth. At the mouth 
of this river the purple felsite is the cliff rock, and at E is faulted against 
'See illustrations to Clarence King’s 40th Parallel Report, vol. i, plate xxi, and vol. ii, plate xxiii. 
