■316 J. Parsons — Brown Mica developed from Aug He. 



Giimbel has figured a somewhat similar variety from the numrau- 

 litic series of Kressenberg, Bavaria, which he has referred to 

 d'Archiac's OpercuUna canalifera} 



The specimens from Sinai measure -2V inch (7 mm.) in the 

 diameter of the shell, and its thickness is ^V inch (-5 mm.). 



Schwager's specimens were obtained from the white clay of Aradj 

 (Mokattam Series), Egypt. 



Coll. Geol. Surv. Egypt, No. 4,112, Box No. 21. ? Bartonian 

 Series (Upper Eocene) or ? top of Mokattam Series (Middle Eocene) : 

 beach deposit, Jebel Abyad, Sinai. Common. 



Heterostegina, d'Orbigny [1826]. 



Heterostegina depressa, d'Orbigny. (PI. XIII, Fig. 7a.) 

 Seterostegi7ia depressa, d'Orb., 1826 : Ann. Sci. Nat., vol. vii, p. 305, No. 2, 

 pi. xvii, figs. 5-7 ; Modeles, No. 99. 



The specimens now under description are very distinctly coiled 

 at the commencement, and in general aspect resemble those speci- 

 mens which belong to the microspheric form ; and which are rare in 

 our modern marine dredgings. This species is more commonly found 

 in the later Tertiaries, but it is not entirely absent from the base of 

 the Eocene. 



Coll. Geol. Surv. Egypt, No. 3,902, Box No. 15Z. Libyan Series 

 (Lower Eocene) : Jebel Krer, Sinai. Eare. 



[To be continued in ottr next j^umber.) 



VI. — The Development of Brown Mica from Atjgite by 

 Eeaotion with Felspathic Material. 



By James Parsons, B.Sc. (Lond.). 



THE conversion of augite or hornblende into brown mica appears 

 to take place from three causes : — 



1. By crushing of pyroxenic minerals and felspar, as has been 

 observed by Professor Bonney in sundry schists of the Alps.- 



2. By the intrusion of felspathic material into a rock containing 

 augite or hornblende, as in the Harz, where granite invades diabase, 

 converting the augite into biotite.^ A similar change has been 

 observed in the gabbro of the Barnavave Mountain, in the Carling- 

 ford district, the diallage being wholly or in part replaced by biotite 

 owing to the invasion of granophyric magma.* In Sark the intrusion 

 of an aplitic magma into almost pure hornblende has resulted in the 

 formation of biotite.^ 



3. By the corrosive action of a residual felspathic magma on 

 previously consolidated augitic constituents. This method of forma- 

 tion has been noted by Mr. T. H. Holland in the augite-diorites 



1 Abhandl. bayer. Akad. Wiss., vol. x (1868), 1870, p. 664, pi. ii, figs. 112a, b. 



• Q.J.G.S., vol. xlix, p. 104. 



3 K. A. Lessen: Congres luternat. Geol. Coniptes Eendues de la 4™'= session, 

 88, p. 184. 



* Sollas: Trans. Roy. Irish Acad., vol. xxx (1894), p. 477. 

 5 Bonney: Q.J.G.S., vol. xlviii, p. 122 et seq. 



