RED ROCKS OF DULUTH. 271 
patches and again in little strings running through and through the feld- 
spars, in the usual manner of secondary quartz. A number of these small 
patches of quartz lying near each other will polarize together, showing that 
they are part of one individual. Moreover, the same is true of the larger 
quartz areas, and numbers of small particles lying near them, so that all 
of the quartz is considered to be secondary. This secondary quartz is fre- 
quently scattered through the feldspars in such a manner as to present the 
appearance of graphic granite, and again it is arranged in irregularly radi- 
ating lines. Chlorite is often present as an alteration-product of both feld- 
spars and hornblende. No base finer than the rest of the rock was observed, 
so that the name should apparently be syenite, the quartz being taken as 
secondary. Since the hornblende is probably uralite, as in the similar rocks 
of other parts of the extent of the formation, the rock is probably an augite- 
syenite. We have in this rock precisely the same as is found in many peb- 
bles of the porphyry-conglomerates of Keweenaw Point, and such as is found 
massive again along other portions of the Minnesota coast. 
Another kind, less common than the foregoing, presents a red matrix 
with little green or black in it, but numerous facets of red feldspar difficult 
to distinguish from the matrix, save in certain positions. This is a true 
“oranitic porphyry,” standing between the granites and felsites. Under 
the microscope it appears originally to have been composed of a minutely 
crystalline base; but the whole is now saturated through and through with 
secondary quartz. 
Less common than either of the foregoing, but still forming quite large 
patches in the gabbro, is another red rock which presents to the naked eye 
an aphanitic light-red matrix, in which minute orthoclases are very sparsely 
scattered. Underneath the microscope it shows a nearly white matrix so 
thoroughly saturated with secondary quartz that it is often difficult to tell 
its exact original nature. The quartz is arranged in arborescent clusters 
and in crossing forms, and all of a cluster will polarize together. Numer- 
ous black particles, some of them undoubtedly magnetite, others more mi- 
nute and hair-like, are contained; also porphyritic crystals of orthoclase and 
augite. The whole section is dotted with minute brownish particles. One 
or two minute porphyritic quartzes were observed. The rock is plainly 
