STRUCTURAL GEOLOGY. 55 



Igneous rocks. Pre-gabbro eruptives.] 



Animikie revolution in the northeastern part of the state. Such sediments might 

 constitute the transition link of the Animikie fragmentals in the central part of the 

 state to the lower horizons of the Puckwunge (Potsdam). Still, it is at present 

 impossible to refer this red sandstone and shale, as developed in several of the deep 

 wells in the central part of the state, with certainty to either horizon. 



The igneous rocks of the Taconic. The igneous rocks of the Taconic have been 

 included en masse in the Keweenawan, and it is best to employ that designation with 

 its original definition, rather than restrict its significance to include only the erup- 

 tives which accompanied and followed the clastic base of the Keweenawan.* But in 

 order to avoid being misunderstood, and to insure detiniteness to our desciptious, 

 these eruptives will be divided into two general groups, with new names, viz.: 



Cabotian : To include the gabbro and cotemporary accompanying red rock and 

 their surface lavas and all their dikes and sills. These igneous rocks are associated 

 with, but in date followed, the Animikie clastic rocks, and preceded the Puckwunge 

 conglomerate, their debris being found in that conglomerate. This term is the desig- 

 nation given by Buchette to the mountain ranges on the north shore of lake Superior, f 



^fan^icll(: To include the cotemporary igneous rocks that accompanied and 

 followed the Puckwunge conglomerate, whether surface flows, sills or dikes, extending 

 to the top of the Keweenawan igneous rocks. This name is that of a river entering 

 lake Superior in Minnesota. 



The elastics that are associated with the Cabotian igneous rocks are of Animikie 

 age, to the bottom of the Puckwunge conglomerate. Those which were cotemporary 

 with the Manitou are of late Potsdam age. These graduate upward to sandstones 

 free from igneous rocks, and to pure quartz sandstones, viz., the Hinckley and St. 

 Croix sandstones. 



Possible pre-gabbro eruptives. There are some reasons, mentioned under the 

 paragraph which treats of the clastic strata of the Taconic, for believing there was 

 some, probably gentle, eruptive action, after the substantial close of what we know 

 of the Animikie, earlier than the convulsion which gave origin to the gabbro; and, 

 further, that these eruptives were accompanied by some detrital accumulation. In 

 the absence of definite facts that bear unmistakably on this point, it will be better 

 to omit here further reference to these problematical eruptives, and to begin this 

 description of the Cabotian igneous rocks with an account of the gabbro. 



The Cdhntniii epoch of eruption. In superficial area, amounting to about 2,300 

 square miles within the state, in the associated problems of origin, date of intrusion, 

 structural relations with older and also with more recent rocks, in variations of its 



* This restriction was once suggested by the writer. Anie,-i,-nn (Ji-i,lnf/ist, xvi, 383, 1885. 



t " During this distance the Kt Louis river, a stream of prime magnitude, bursts through the high trap range of what Bonchette 

 calls the Cabotian mountains, being a continuation of the upheavals of the north shore of lake Superior." SCIIOOLORAKT, in fiinti- 

 muni/ .\~armlin- ,>fnn I-:, /:!r, ,;,/,,, -it /:' i /irililiini In Ihi- Hinirri-x ni tJic .V/.w'.<i/>;>i' tf/i-rr. 1851, p. 110. 



