G. W. Card— Flexibility of Rods. 121 



early writers referred to it as "Itacolumite"; a list of such references 

 will be found in Zirkel's "Lehrbuch der Petrograpbie" (Bd. ii. p. 484:). 



Subsequently similar rocks, also occasionally exhibiting flexibility, 

 were discovered in other parts of the world, more especially in the 

 Southern States of North America, and, at a later date, in India. 



The term " Itacolumite " has been applied to the rock itself by 

 most writers ; Tuomey, who restricts it to the flexible variety, is 

 an exception. The accounts given of the rock are fairly consistent, 

 all agreeing in describing it as a stratified and granular variety of 

 qnartzite ; frequently, especially in the case of the Brazilian and 

 Carolina specimens, a good deal of mica occurs upon the bedding- 

 planes. 



Coming now to the terms applied to the flexible portion, " Sand- 

 stone " may be objected to on the ground that flexibility is at present 

 unknown in unaltered siliceous sedimentary rocks, but Prof. Derby 

 has pointed out ^ that there is no reason why it should not occur in 

 sandstone. 



Almost without exception the flexibility sometimes exhibited by 

 " Itacolumite " has been ascribed to the quartz grains being en- 

 veloped by a flexible mineral variously referred to as mica, sericite, 

 talc, or chlorite. As exceptions I may mention Klaproth and Prof. 

 Haughton, both quoted ^ by Mr. Oldham, and von Hausmann, referr 1 

 to by Zirkel.^ Leibner, in the report on South Carolina bef 

 quoted, devotes a good deal of attention to the cause of flexibility, 

 and considers that four conditions must be fulfilled, viz. : 



1. " Fineness of grain." 



2. " Sufficient admixture of mica or talc." 



3. "Delicately laminated structure." 



4. "Certain degree of compactness in the constituents of each 

 lamina." 



In this way he explains the very local occurrence of portions, 

 since all the conditions must be fulfilled simultaneously. It would 

 seem as if the flexible-mineral theory was at first nothing but a 

 suggestion, but Leibner definitely gives his assent to it. Leaving 

 the older views — which are still given in most text-books — two im- 

 portant papers have been lately published by Mr. Oldham,* and by 

 Prof. 0. Miigge respectively,^ which propound quite difterent views 

 as to the cause of flexibility. To these may be added a third by 

 Prof. Derby ^ on the mode of occurrence of the Brazilian specimens. 

 Prof. Derby's paper deals with the very local occurrence of flexible 

 portions in the Itacolumite Series ; the same mass of rock may 

 contain "massive and schistose, compact and friable, non-flexible 



1 Am. Journ. Sci. xxviii. 1884, p. 205. 



^ Rec. Geol. Surv. India, vol. xxii. pt. 1, 1889, p. 53. 

 ' 3 Lehrbuch der Petrographie, 1886, Bd. 11, p. 482. 



* On Flexible Sandstone or Itacolumite, E. i). Oldham, A.R.S.M., Records Geol. 

 Sm-v. India, vol. xxii. pr. 1, 1889. 



^ Ueber " Gelenksandstein " aus der Umgegend von Delhi 0. Miigge, Neuen 

 Jahrbuch, Bd. i. 1887. 



^ On the Flexibility of Itacolumite, Orville A. Derby, Am. Journ. Sci. vol. xxviii. 

 1884. 



