SUMMARY 339 



river at least 6,000 feet of lava beds and tuff deposits occur. On the 

 west side of the syncline the series is sharply defined by a fault line 

 from which the lava flows dip eastward at a sharp angle. Along the 

 Kettle river the dip is as high as 50 degrees, and on the Snake it reaches 

 an average inclination of 67 degrees. These flows, as the outcrops on the 

 two streams from 7 to 10 miles apart are correlated, attain a thickness 

 of 20,000 feet, comparable in extent with the Keweenawan of the north 

 shore of lake Superior along the Minnesota coast, the south shore on 

 Keweenaw point, and the more extreme eastern beds of Maimaisne 

 point. 



4. The Snake River outcrops at Chengwatana are called the Chengwa- 

 tana series. They exhibit a series of 65 lava flows and 5 conglomerate 

 beds in continuous succession, with neither the base nor the summit 

 exposed. The lava flows are typical, presenting the finely crystalline 

 base, mediumly crystalline middle part, and amygdaloidal upper part 

 strongly in evidence. The visible flows can be traversed for 3,500 feet 

 in vertical thickness, while the entire series probably attains the thick- 

 ness named in paragraph 3. 



5. The petrographic characters are described as exhibited in the two 

 groups of rocks, elastics and eruptives. The former are (1) conglomer- 

 ates, with the specific characters incident to this region, and (2) breccias 

 and tuffs lying between the lava flows and usually thoroughly consoli- 

 dated. The latter are diabases, either compact or amygdaloidal, and 

 usually of the ophitic type. The prevailing colors are dark. In places 

 the mottled appearance prevails peculiar to ophitic diabases in the Lake 

 Superior region. A porphyritic habit is here and there, and a marked 

 concentric weathering is seen in a few of the flows. 



6. In discussing the source of the eruptives the failure to discover vol- 

 canic vents is stated, and the probability of a series of vents located 

 along the fault line is named. The plausibility of this explanation is 

 seen as the extent of the fault line is considered in connection with the 

 large number of consecutive flows which can be counted along the west 

 side of the syncline at several places, 65 being the number seen at Cheng- 

 watana. Localities of modern volcanic activity are mentioned in sup- 

 port of the view that the lava came from fissures which discharged 

 material at many points. 



7. The development of the fault line through a series of fissure erup- 

 tions and the line of weakness incident thereto is suggested. The tilting 

 of the Keweenawan rocks to a sharply inclined attitude can be traced 

 for more than 100 miles from the Lake Superior region southwestward. 



