414 



REPORT OF NATIONAL MUSEUM, 1904. 



Of greater importance was the fact, first announced in his report for 

 1847-48, but here brought out in detail, that underlying his lower 

 magnesian limestone (Chazy) there were at least six different trilobite- 

 bearing beds, separated by from 10 to 150 feet of intervening strata. 

 Previous to this no remains of this nature had been reported from any 

 American strata older than the Canadian period of the Lower Silurian. 

 These trilobite-bearing strata, it should be noted, were found resting 

 immediately upon the primal rocks and hence formed the true base of 

 the zoological series in the Mississippi Valley. 



On the title page of this volume appeared the cut of the horizontally 

 jointed trap of Lake Superior that has long done duty in the text- 

 books of Le Conte and others (see tig. 47). 



Fig. 47.— Trap dikes on Lake Superior. (After D. D. Owen.) 



In accordance with an act of Congress approved March 1, 1N47. Dr. 

 C. T. Jackson was appointed by the Hon. R. J. Walker, then Secre- 

 tary of the Treasury, to make a geological survey of that portion of 

 Michigan lying 1 south of Lake Superior and north and 



Work of C. T. fe J ° ,«■■!. ■ 



Jackson in northwest of Lake Michigan. As in previous opera- 



Michigan, 1847. . *, • . 



tions of like nature by Owen, the object of this survey 



was to ascertain which of the lands should be classed as mining lands 

 and which agricultural. Jackson spent two seasons in the prosecution 

 of this work and then resigned, for reasons which seem to have been 

 in part personal and in part political, the completion of the work being 

 confided to his assistants, J. W. Foster and J. D. Whitne}\ 



Jackson's report was published as House Document No. 5, Thirty- 

 first Congress, first session, 1849. It comprised upward of 560 pages, 

 with 19 plates, geological maps of Keweenaw Point and Isle Royal, 

 and three sections of mines. The eruptive rocks of Keweenaw Point 

 were described as having been intruded through linear caverns or 

 fractures in the superincumbent rock which they had frequently over- 

 flowed, so as to rest unconformahly on their strata. Here is again 



