THE ORIGIN OF CERTAIN SILICEOUS ROCKS 3^9 



Crosby thinks some of the jaspers and quartzites are the 

 siliceous oozes of the deep sea, that is, that they are of organic 

 origin. 1 



Wadsworth at first believed in an eruptive origin for the 

 jaspilites. 2 This view he modified later. 



Julien believes in the mechanical sedimentary origin of the 

 jasper iron ores of the Marquette region, and it is to be pre- 

 sumed that he considers the jaspers mechanical sediments also. 3 



Irving concluded that the jaspers were formed by silica 

 replacing dolomite or calcite. 4 



Reyer thinks the sediments associated with the iron ores of 

 Michigan are derived partly from decomposed igneous rocks and 

 partly from organic remains. 5 



Branner has suggested that the compact novaculites may be 

 metamorphosed cherts. 6 



Comstock considered many of the Arkansas novaculites to 

 be hot water deposits. 7 



Julien thinks amorphous silica is produced by marine organ- 

 isms, or by precipitation from solutions. 8 



Griswold is of the opinion that novaculites are simply meta- 

 morphosed, fine grained, mechanical sediments. 9 



N. H. and H. V. Winchell believe in the theory of chemical 

 precipitation for the jaspilites. 10 



James E. Mills is of the opinion that certain quartzites of the 



1 Proc. Bost. Soc. Nat. Hist, Vol. XX, pp. 167-168, 1878-1880. 



2 Bull. Mus. Comp. Zool., Vol. VII, p. 30, 1880 ; Proc. Bost. Soc. Nat. Hist., Vol. 

 XX, pp. 477-478, 1878-1880; Bull. Mus. Comp. Zool. Harvard Col., Geol. Ser.'I, 

 Vol. XVI, pp. 331-565, 1884. 



3 Genesis of the Crystalline Iron Ores. By A. A. Julien. Eng. Mining Jour., 

 Vol. XXVII, pp. 81-83, 1884. 



" Am. Jour. Sci., Vol. XXXII, p. 255, 1886. 



5 Geologie der amerikanischen Eisenlagerstatten. E. Reyer. Oest. Zeitsch. f. 

 Berg. u. Huttenwesen, XXXV, Nos. 10 and 11, 1887. 



6 Ann. Rep. Geol. Surv. Ark., Vol. I, p. 49, footnote, 1888. 



7 Ann. Rep. Geol. Surv. Ark., Vol. I, pp. 95. 129, 1888. 



8 Proc. A. A. A. S., Vol. XXVII, pp. 311-340, 1889. 



9 Ann. Rep. Geol. Surv. Ark., Vol. Ill, pp. 164, 194, 1S90. 



10 The Iron Ores of Minnesota. By N. H. and H. V. Winchell. Minneapolis, 

 1891, p. 74- 



