GEOLOGICAL AND NATURAL HISTORY SURVEYS. 177 



in connection with the greenstone. He thinks the chionology de- 

 ducible from tlie dikes shows that " the mineral region of the Upper 

 Peninsula is strictly confined to only the outer portion of the rocks 

 of a single epoch " (p. 24). 



These veins and dikes of greenstone have no uniform bearing. In 

 width they vary from a mere line to 50 or GO feet. Tliey disintegrate 

 more rapidly than tlie contiguous rock. 



On the north coast of Lake Superior granitic and syenitic rocks 

 also occur, but " more frequently they are flanked on the south by 

 greenstone, with occasional narrow bands of sandstone; thus pre- 

 cisely reversing the magnetic order of these rocks upon the south " 

 (p. 25). 



2. Trap roehs. — These flank the primary rocks on the north and 

 northwest, and form a series of ranges of hills stretching generally 

 southwest and northeast, and rising from 300 to 900 feet above the 

 lake. They are less knobby than the primary, but the contrast grows 

 stronger a^ we recede from the primary. In lithological constitu- 

 tion the contrast follows the same rule. The range beginning at the 

 extremity of Keweenaw Point stretches to the Ontonagon Eiver, 

 gradually receding from the coast. West of this it becomes con- 

 founded with the northerly portion of the Porcupine Mountains. 

 West of the mountains the same range is continued to the Montreal 

 River, where it is but 2 miles distant from the lake. West of the 

 mountains is also a second range 12 to 15 miles inland. All the 

 northwest part of He Roj'ale is of the same rock. 



The age of the trap rocks is subsequent to that of the slates and 

 quartzites denominated metamorphic. The term " trap rocks " in- 

 cludes both greenstone and amygdaloid. Greenstone includes not 

 onh'^ those rocks composed of feldspar and hornblende but also those 

 which m.ight more strictly be regarded as altered syenite, syenitic 

 granite, hornblende rock, and augitic rock. Amygdaloid is the same 

 in a certain structural condition. 



The rocks of the outer or northwestern range of hills, which were clearly the 

 last of the series of uplifts, bear more unequivocally the evidence of igneous 

 action. On the ^outh flanks of these hills the rock is invariably very compact 

 jrreenstone, while upon the northwestei'ly line it is almost equally invariably 

 an iiniyjrrlaloicl. or at least has an amygdaloidal structure. The cause of this 

 is found in the fact that the uplift of the rocks of this range of hills was wholly 

 rpon the southeasterly side, and while the rocks of this i)ortion were in a solidi- 

 fied state: or, in other words, that a point in Lake .Superior may be regarded 

 as the fixed axis of the uplifted mass. 



This view is confirmed by the undisturbed condition of the sedi- 

 mentary rocks on the south or southeast, and their highly dis- 

 turbed condition on the northwest, while the angle of dip diminishes 

 as we recede from the range. These strata have been traversed by 



