840 A TREATISE ON METAMORPHISM. 



not destroy the original sedimentary banding, but, on the contrary, empha- 

 size it. The striking banded appearance of actinolitic and griineritic rocks 

 is one of their most characteristic features. 



The development of the actinolitic and griineritic marbles and of the 

 actinolitic, griineritic, and magnetitic quartz-rocks is very greatly promoted 

 by the presence of igneous rocks. Indeed, in the Lake Superior region, 

 where iron silicates have been produced in large amounts from the iron- 

 bearing carbonates and hydrous iron silicates over extensive areas, intrusive 

 rocks, in large masses, seem to be invariably present. At the west end of 

 the Penokee range in Wisconsin, and at the east end of the Mesabi district 

 in Minnesota," the entire iron-bearing formations have been changed to 

 silicate-bearing rocks, and directly in contact with them- — indeed, intrusive 

 in them in a complex maimer — are great batholithic masses of the basal 

 gabbro of the Keweenawan. Also in the Mesabi district granite has been 

 intruded in great quantities. In the Marquette district of Michigan, east of 

 Negaunee, dolerite is a very abundant intrusive within the iron-bearing 

 formation, and there in the iron-bearing formation the silicate rocks are 

 again found, although not so abundantly as in the Mesabi and Penokee 

 districts. 6 Not only do the great areas of the silicated rocks occur where 

 there are great masses of intrusives, but intrusives are sure to be found 

 where the silicated rocks appear locally abundant, as, for instance, adjacent 

 to Humboldt, Mich., in the Marquette district. Apparently in the Lake 

 Superior region the iron-bearing formations were not sufficiently deeply 

 buried, or subjected to sufficiently strong orogenic movements to cause the 

 silication of the carbonates to take place on an extensive scale for those 

 reasons alone; but where these metamorphic conditions were reenforced 

 and intensified by intrusive rocks, there the transformation took place. If 

 any case of metamorphism ought to be called contact metamorphism, the 

 development of the silicated rocks from the iron-bearing carbonates should 



«Van Hise, C. R., and Irving, R. D., The Penokee iron-bearing series of Michigan and Wisconsin: 

 Mon. U. S. Geol. Survey, vol. 19, 1892, pp. 257-260. Grant, IT. S., Sketch of the geology of the east- 

 ern end of the Mesabi iron range in Minnesota: Eng. Year Book, Univ. of Minnesota, 1898, p. 58. 

 Van Hise, C. R., and Leith, C. K., The iron-ore deposits of the Lake Superior region; section on the 

 Mesabi district: Twenty-first Ann. Rept., U. S. Geol. Survey, pt. 3, 1901, pp. 359-360. Leith, C. K., 

 The Mesabi iron-bearinj; district of Minnesota: Mon. U. S. Geol. Survey, vol. 43, 1903, pp. 159-164, 

 182-188. 



* Van Hise, C. R., and Bayley, W. S., The Marquette iron-bearing district of Michigan: Mon. 

 U. S. Geol. Survey, vol. 28, 1897, pp. 380-381. 



