368 ORVILLE A. DERBY AND J. C. BRANNER 



probably be often better than the " commercial," and the prepara- 

 tion and examination of residues more rapid and satisfactory 

 than that of microscopic sections. 



Orville A. Derby. 



Sao Paulo, Brazil, April 6, 1898. 



II. ON THE ORIGIN OF NOVACULITES AND RELATED ROCKS. 



The preceding part of this paper was recently received by 

 me from Professor O. A. Derby as embodying the results of his 

 examination of novaculites by a method hitherto not employed 

 with those rocks. It should be added that in a former letter he 

 sent me the results of a study by this same method of an impure 

 cherty Carboniferous limestone from Tiete, Sao Paulo. In this 

 limestone he found by crushing and washing, rolled quartz, 

 zircon, rutile, garnet, tourmaline, etc. 



These notes I have taken the liberty to publish without con- 

 sulting Professor Derby in order that the results of his work 

 may become available for others who may study this interesting 

 and important group of rocks. 



It is remarkable that so many different views have been held 

 regarding the origin of novaculites and of the closely related 

 highly siliceous rocks known as jaspilites and sometimes as 

 jaspers, siliceous shales, etc. 



The following theories have been advanced to explain the 

 origin of these rocks : 



Foster and Whitney seem to have regarded the jaspers of 

 Michigan 1 as segregations of eruptive origin. Owen considered 

 the Arkansas novaculites as metamorphosed sandstone. 2 



Kimball believed the Marquette iron ores and their associated 

 jaspers to be metamorphic rocks of sedimentary origin, presum- 

 ably mechanical. 3 



1 Geology and Topography of the Lake Superior Land District. Pt. II. The 

 Iron Region, pp. 67-69, 1851. 



2 Second Geol. Surv. Ark., pp. 23, 25, i860. 



3 Am. Jour. Sci., Vol. XXXIX, p. 303, 1865. 



