10 BULLETIN OF THE 



mense consolidated lava-stream, except that the vesicular structure is 

 wanting. To the north, the surface of the igneous rock is bare ; but, 

 on the eastern side, it is covered in places with a rudely stratified mass, 

 which appears to have been deposited in the inequalities of the pre- 

 existing surface. It resembles a volcanic sand, or ash, portions of it 

 being composed of a scoriaceous mass of a light-brown color, and reticu- 

 lated with numerous veins of a white mineral, portions of which are cal- 

 careous, and others silicious." {I. c, pp. 121, 122; see, also, pp. 18 

 and 92.) 



This rock was thought to be of prior origin to the sandstone. Tlie 

 sandstone was regarded as Potsdam, and as the same rock as that 

 on Keweenaw Point. It was pointed out that this sandstone rests 

 nearly horizontally on the water-worn edges of the nearly vertical 

 quartzite. (l. c, pp. 122, 123.) The iron ores were regarded as of 

 igneous origin, forming intrusive masses and ovei'flows, principally the 

 latter, like a lava, but consolidated under pressure of a deep ocean. 

 Sublimations of the iron occurred while the denudation and deposition 

 of the eruptive masses that wei*e by the shore-line aided in making the 

 different formations. After this series of igneotis and aqueous ore- 

 beds were laid down, " the whole series of beds, slaty, quartzose, ferru- 

 ginous and trappean, were elevated, and, in all probability, folded, per- 

 haps at the epoch of the elevation of the granite ranges on the north 

 and south of the ferriferous belt of the azoic system." {I. c, pp. 

 50-69.)* 



Dr. J. J. Bigsby regarded the granite as igneous, and taught that the 

 metamorphic rocks had been " upheaved and altered by the intrusion 

 of igneous rocks in instances innumerable." t Professor "Whitney, in his 

 ''Metallic Wealth of the United States," (Philadelphia, 1854,) again ad- 

 vocated the igneous origin of the iron ores in this district, as well as in 

 some other localities. J 



In 1854 there was published by Henry R. Schoolcraft, in Philadelphia, 

 a work entitled " Summary Narrative of an Exploratory Expedition to the 

 Sources of the Mississippi River, in 1820 : resumed and completed by the 

 Discovery of its Origin in Itasca Lake, in 1832. By Authority of the 

 United States. With Appendices, comprising the Original Report on 



* Am. Jour. Sci., (2,) XVIL 11-38, 1854 ; XXIL 305-314. 



t Edinburgh New Phil. Jour., 1852, LIII. 55-62. 



} Pp. 37, 429-437, 477, 478. See also Proe. Am. Assoc. Adv. Sci., 1855, 

 209-216; Miuincr Magazine, (New York, 1856,) VII. 67-73; Am. Jour. Sci., 

 (2,) 1856, XXII. 38-44. 



