REVIEWS 169 



The Keweenawan rocks consist of gabbro, diabase, and granite, all of which are 

 intrusive into the rocks with which they come into contact. The north edge of the 

 gabbro runs diagonally across the east end of the district from southwest to northeast, 

 resting upon the edges of each of the members of the Upper Huronian series, and at 

 Birch Lake against the Lower Huronian granite. North of the gabbro margin, in 

 Range 12, are isolated exposures of diabase which may represent sills associated with 

 gabbro intrusion. The granite forms the crest of the Giant's Range through Ranges 

 12 and 13. This granite has not heretofore been discriminated from the Lower 

 Huronian granite. The exomorphic effect of the gabbro and the granite upon the 

 Upper Huronian series has been profound. 



J. Morgan Clements. "The Vermilion Iron-Bearing District of Minne- 

 sota." Mo7iograph XLV, U. S. Geological Survey, 1903. 



Clements describes the geology of the Vermilion iron-bearing district of Minne- 

 sota. Elaborate general and detailed maps, accompanying this report, are based on 

 field work by Clements, Van Hise, Bayley, Merriam, and Leith. 



The district ranges from two to eighteen miles in width, and extends from a little 

 west of Lake Vermilion in a direction a little north of east to Gunflint Lake on the 

 international boundary, a distance of about one hundred miles. 



The rocks of the district are described under the headings "Archaean," "Lower 

 Huronian," and "Upper Huronian," representing series separated by marked uncon- 

 formities. 



The Archaean of the Vermilion district is'divided into three formations, as follows, 

 given from the base up : the Ely greenstone, the iron-bearing Soudan formation, and the 

 granites of Vermilion, Trout, Burntside, Basswood, and Saganaga Lakes. 



The Ely greenstones consist of basic and intermediate igneous rocks widely dis- 

 tributed in anticlinal areas, as shown by the distribution of the overlying sediments. 

 They were originally rocks corresponding in character to intermediate andesites and 

 basic basalts. They have been extremely altered, but retain in many cases in striking 

 perfection the original structures, such as ellipsoidal parting, and spherulitic and 

 amygdaloidal structures. A study of their various textures and structures shows that 

 these greenstones are unquestionably of igneous origin, and are largely of volcanic 

 character. Many of them have been rendered schistose by pressure. The greenstones 

 have also been strongly affected by the contact metamorphism due to the intrusion of 

 great granite masses. As a result of this intrusion, there have been produced from 

 the greenstones amphibole-schists, which form a marginal facies of the greenstones, 

 lying between them and the adjacent granites. The greenstones have also been met- 

 amorphosed by the Duluth gabbro of Keweenawan age, and granular rocks have thus 

 been produced which in most cases show the original textures of the greenstones, but 

 contain also a development of fresh biotite, hypersthene, brown-green hornblende, 

 and magnetite. 



The Soudan iron formation is widely distributed in the western part of the dis- 

 trict, but is practically wanting in the eastern half. It is found mostly in narrow belts, 

 which consist largely of greenstone so intimately associated with the iron formation 

 that it has been impossible to separate them on the map. The formation consists of 

 (i) a very subordinate fragmental portion made up of some conglomerate, clearly rec- 

 ognizable as having been derived from the underlying greenstones, grading up into 

 sediments of finer character; and (2) lying above this fragmental portion, the iron- 



