GEOLOGICAL EXPLORATIONS AND LITEEATDItE— 1S73. 57 



miles. ♦ * * "While some of tlie beds present litliological ebaracters so constant 

 that tbey caji be identified wberever seen, others undergo ,i;reat changes. JLirble 

 passes into quartzite, which in turn graduates into novaculite; diorites, almost por- 

 phyritic, are the ecpiivalents of soft magnesian schists. * * * The total thickness 

 of the whole series in the Marquette region is least at Lake Superior, where oidy 

 the lower beds exist, and greatest at Lake JMichiganime, where the whole nineteen are 

 apparently jiresent, and may have an aggregate thickness of 5,000 feet. (Pp. S3-S4.) 

 With regard to the associations of the various ores, it may be said that magnetic 

 and specular ores are often found together, as are also the specular and soft hematite 

 ores; but so far the magnetites and hematites have not been found in Juxtaposition. 

 If we suppose all our ores to have once been magnetic, and that the red specular was 

 first derived froni the magnetite, and the hydrated oxide (soft hematites) in turn from 

 it, we have an hypothesis which best explains many facts, and which will be of use to 

 the explorer. (I'p. 220-221.) 



Besides the niaguetic charts, three g'eoh)o-ical maps pertaining to the 

 Marquette district accompany tlie report. One represents the general geol- 

 ogy of the entire Upper Peninsula (see PI. Ill), the second is a detailed 

 map of the whole of the Marquette district, and the third is a lai'ge-scale 

 map of Reiniltlic Mountain. The northern boundary of the iron rocks is 

 placed much farther north by Brooks than it is in this monograph. Brooks 

 included with his Huronian all of the greenstone-schists north of the iron- 

 bearing' rocks, and made their contact with the granite the l)Oundary line 

 between the Marquette iron-bearing rocks and the Laurentian. These 

 schists are Group XIII of Brooks's series, and are regarded by him as 

 high in the series. In this volume the}' are placed below the whole of the 

 ii'ou series. 



Several appendixes are added to Brooks's report and published as 

 Vol. II of the Michigan State survey. Some of them are of great scientific 

 interest. Those of Julien and of Charles E. Wright are the first articles in 

 which the lithological features of the Marquette rocks are described in detail. 



Julien, Alexis A. Lithological descriptions, etc., of 259 specimens of the 

 Huronian and Laurentian rocks of the Upper Peninsula. Geo!. Surv. of Michigan, 

 1869-1873, Appendix A, Vol. II, New York, 1873, 197 pages. 



The aim of Julien's report, in the words of its author, as given in the 

 letter of transmittal addressed to Brooks, "is but a provisional one, viz, to 



