638 FRANK F. GROUT 



latest flow exposed. About three strikingly peculiar types occurred 

 in a few isolated outcrops and not elsewhere in the series. A line 

 connecting the outcrops of a single type proves to run due northeast, 

 conforming to the observed strike near by. Such data furnish the 

 best evidence in Minnesota that a single flow is continuous for over 

 25 miles. The red laumontite pseudo-amygdaloid is so traced, and 

 within two miles across the strike a highly porphyritic type runs 

 parallel with it. 



Geographically the area may be briefly summarized. The Taylors 

 Falls and Snake River districts are discussed in earlier publications, 

 but the remarkable freshness of one of the Snake River rocks is 

 worthy of further mention. In the new area studied there are a few 

 local variations. Most of the exposures are along creek banks. 

 The rock types seen on the western side of the syncline on Kettle 

 River are found due northeast on the smaller creeks. These types 

 include the red pseudo-amygdaloid and a highly porphyritic rock on 

 each creek. Along the lower parts of the eastern creeks the types 

 vary somewhat, but seem to be local modifications rather than evi- 

 dence of extensive changes of nature. 



Economic. — Copper is found in small amounts in test pits on all 

 the easily accessible streams. Its association varies — on Snake 

 River where most explored, it is in laumontite, and on Kettle River, 

 in prehnite. Some nice specimens are produced. In connection 

 with the chemical side of this study of rocks, it was discovered that 

 the fresh trap rock was the source of the copper. Indications are 

 that the original lavas contained from o . 01 to o . 03 per cent of copper. 



PETROGRAPHY 



Review. — Descriptions of Keweenawan rocks written between 

 thirty and forty years ago have not been seriously questioned, though 

 the method of naming rocks has developed greatly. All attempts, 

 including this one, result in complex statements referring partly to 

 original conditions, and partly to present altered ones. A summary 

 and correlation of rock-names, in the literature, is given by A. N. 

 Winchell,^ so that the full list will be omitted. 



Chemical data have been presented, in addition to field and micro- 



^ Journal of Geology, XVI, 765. 



