542 THE GEOLOGY OF MINNESOTA. 



[Pipestonc. 



its distinctive bedded structure, which may be seen when examined micro- 

 scopically in polished thin sections. Indeed it seems to have a laminated 

 structure; and the different shades of color appear sometimes to be due to 

 openings and fissures produced in the red clay becoming filled with sedi- 

 ment of a lighter color. The following analyses* have been made of this 

 substance. It is not truly a mineral but an indurated clay, and its chemi- 

 cal composition varies in consequence. Analysis No. 1 shows the results 

 obtained by Dr. C. T. Jackson from the sample procured by George Catlin 

 in 1837. Nos. 2 and 3 were obtained by the writer in 1877, and were ana- 

 lyzed by S. F. Peckham, the former being of a red color and the latter of a 

 pinkish color. No. 4 was obtained by Mr. Upham from "the palisades" in 

 Minnehaha county, Dakota. It is of a very light color, and is known as 

 "chalk rock." Its color seems to have been derived accidentally, in situ, as 

 it is in the line of extension of a bed seven feet thick of a mottled variety 

 of pipestone. No. 5 was obtained by Prof. R. D. Irving at Devil's lake, 

 Wisconsin, and is of a lilac-brown color, analyzed by Prof. W. W. Daniells. 



1. 8. 3. 4. 5. 



Water 8.40 7.44 6.48 9.60 2.50 



Silica 48.20 57.43 58.25 50.40 62.16 



Alumina 28.20 25.94 35.90 33.30 29.67 



Magnesia 6.00 .... 0.17 



Peroxide of iron 5.00 8.70 2.80 4.17 



Peroxide of manganese. . 0.60 



Lime 2.60 .... 0.60 0.16 



Alkalies... 4.10 



99.00 99.51 100.63 100.97 99.36 



This substance is found at various places in Minnesota and Wisconsin. 

 Indeed it seems to graduate into red shale, and becomes in that form an 

 important constituent of the formation in which it is found.f It seems to 

 be only when this formation is greatly indurated that the inclosed shale 

 beds are hardened to the condition of pipestone. In cases of greater meta- 

 morphism its heaviest deposits have been converted apparently into red 

 felsite or quartz porphyries. 



Although this substance has usually a red color, like that which pre- 



*See page 62; also Am. Jour. Sci. (1). xxxv. 318; Dana's System of Mineralogy, fifth edition, p. 796; sixth annual 

 report, p. 101; tenth annual report, p. 203 ; Geology of Wisconsin, vol. II p. 510 



The analysis by T. Thomson, given on page 796 of the fifth edition of Dana's System of Mineralogy, is of a pipe- 

 stone obtained fn>m the Indians on the northwest coast of North America. It was of a "lisrht grayish blue color, not 

 much h -rder than gypsum, and did not fuse per se before the blowpipe. Excluding the iron, the composition approaches 

 thnt of an oligoclase. It has no relation to the catlinite." [J. it. Dunain a letter to the writer], AnnalsN. Y. Lyc.HI., 9, 

 1827; Thomson's Mineralogy, I., 2S7, 1836; Dana's Mineralogy, second edition, 1844. p. 591; Id., third ediiion. 



tSee the reports on Blue Earth, Scott, and Hennepin counties, records of deep wells; also Geology of Wisconsin, 

 vol. IV. p. 578; also tenth annual report, pp. 3034. 



