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L. ©. Wooster—The Copper-hearing Series, 463 
Again the distinction between dacite and rhyolite is somewhat 
sharply drawn by a large development of biotite in the dacites, 
whereas as soon as the sanidin makes its appearance as the 
predominant feldspar mica loses its prominence, and in most 
rhyolites of the Basin, although there exist a few marked 
exceptions, mica plays a very subordinate part. 
t might seem natural to suppose that where the basic rocks 
are largely characterized by anorthite and labradorite, the inter- 
mediate rocks by andesine and oligoclase, and the acidic varieties 
by orthoclase, that the trachytes would be represented by at 
least some minor extrusions. Investigation, however, shows 
that the typical trachyte known to occur in other parts of the 
world has never been brought in from this region. Over this 
Wide area with its great variety of volcanic rocks sanidin only 
makes its appearance after quartz has come in as an essential 
constituent among the porphyritic crystals. It may be laid. 
P 
“andesites and trachytes” or “ trachytes and rhyolites.” It 
Seems to us that in future such expressions should be more 
carefully considered, it having been shown that at least in the 
Great Basin such an association of lavas is unknown. 
Our work leads us to believe that trachytes occupy a far 
more restricted position among volcanic rocks than has hereto- 
fore generally been supposed. The independence of rhyolite 
and trachyte from a geological point of view seems quite 
clear, for in regions of widespread volcanic activity it is far 
more frequently associated with andesitic than with trachytie 
eruptions, 
Art. LIV.—Transition from the Copper-bearing Series to the 
Potsdam ; by, L. C. WoosrEr. 
During the summer of 1883, some facts throwing light upon 
the relationship existing between these formations along the 
St. Croix River, Wisconsin, fell under my observation, and, 
though not new in kind;* they may be of interest to those who: 
are endeavoring to find in the East a correlative of the Wis- 
consin Potsdam. . 
In northwestern Wisconsin the Potsdam sandstone has a total 
thickness, from the Laurentian granite below to the Lower Mag- 
nesian limestone above, of about one thousand feet. There are 
two horizons in which fossiliferous remains are especially 
* See vols. i, and iii, Wisconsin Geological Reports. 
