194 GEOLOGY. 



them were largely derived from the igneous rock itself. Their character 

 is such as to indicate that they accumulated rapidly. In spite of its 

 enormous thickness, therefore, it is not clear that this system of rocks 

 occupied more time in its formation than either of those which had 

 preceded, and perhaps not as much. 



In the valley of the St. Croix river, in northwestern Wisconsin and 

 the adjacent parts of Minnesota, there are said to be 65 lava flows and 

 5 conglomerate beds in succession, with neither top nor bottom exposed. 1 

 The aggregate thickness of the exposed part of the system in this 

 locality has been estimated at 20,000 feet. 



Some of the igneous rocks, such as the gabbros of the Vermilion 2 

 and Mesabi 3 regions, are intrusive (laccolithic), while others, as in 

 the St. Croix valley, represent fissure eruptions. The laccolithic gab- 

 bros and granites of the Mesabi region have occasioned extensive meta- 

 morphism in the rocks into which they were intruded. The gabbros 

 (Duluth) of the Vermilion region contain masses of titaniferous mag- 

 netite, believed to have arisen from segregation in the original magma. 

 The magnetite is not of commercial value. Eikes and other intrusions 

 of acid igneous rocks affect the more basic portions, and are of common 

 occurrence. 



Deformative movements. — The sedimentary part of the Keweena- 

 wan series has usually been thought to imply marine submergence, 

 but the sediments may have been accumulated in an interior basin and 

 may be more or less subaerial. If they are marine, their great thick- 

 ness and their derivation mainly from the lavas of the series itself 

 seem to imply crustal warping throughout the period to renew the 

 erosion fields and furnish the necessary sites for their lodgment. If 

 they are merely lodgment deposits on the low lava slopes of the basin, 

 little movement need be assumed during their deposition, and the 

 subsequent deformation need only have been simple and relatively 

 mild, while the thickness is not incredible (see p. 260). Under either 

 view it was probably during and at the close of this period that the 

 Lake Superior syncline (Fig. 77) was formed, in part at least, and 

 the Keweenawan rocks tilted toward its axis, both from the north 

 and south. It is not to be understood that this syncline has remained 



1 Hall, Bull. Geol. Soc. Am., Vol. XII, pp. 313-340. 



2 Clements, Mono. XLV, U. S. Geol. Surv. 



3 Leith, Mono. XLIII, U. S. Geol. Surv. 



