L. F. Spath — Jurassic Ammonites from East Africa. 351 



2. The absence of sulphides and the very small proportion of 

 cassiterite present. 



3. The remarkably coarse texture of the lode-filling. 



The veins of the Kit Hill Mine, Callington, are in some respects 

 similar in character, but they differ in containing cassiterite, 

 tourmaline, and mispickel ; they also occur just below the surface of 

 the granite and not in the clay-slate. There is also some similarity 

 in the veins of the Hemerdon Hill stockwork, which are like those 

 of Kit Hill, with a much smaller proportion of mispickel in the veins, 

 which also contain scorodite. There is again a similar veinstone in 

 the middle lode of the recently reopened Killifreth Mine, St. Day, 

 in which coarse patches of wolfram occur in clear quartz, which is 

 also traversed by veins carrying cassiterite, blende, and mispickel. 



The Castle-an-Dinas lode seems to represent the results of mineral 

 deposition in the upper part of the tin and tungsten zone, the 

 sequence of events having been somewhat as follows : — 



1. Folding and cleaving of the clay-slate. 



2. Intrusion of granite followed by pneumatolytic action, which 

 resulted in the tourmalinization of the slate and the formation of 

 greisen, kaolinized granite, and deposition of cassiterite in the slate. 



3. The opening of the lode fissure. 



4. Filling of the fissure by means of hot solutions or pegmatite- 

 like intrusions. 



On Jurassic Ammonites from East Africa, collected by 

 Prof. J. W. Gregory 



By L. F. Spatii, B.Sc, F.G.S. 



{Concluded from p. 320. ) 

 Genus Sowerbyceras Parona & Bonarelli. 



Sowerbyceras aff. tortisulcato d'Orbigny sp. (PI. I, Figs. 5g-c.) 



1870. Phylloceran n.sp. aff. iorti.ndcato (d'Orbigny) Neumayr, " Jurastud. Ill, 



Die Phylloc. Dogger Malm," p. 345, pi. xvii, fig. 9. 

 1895. Ph. subtortisidcatum (Pomjjeckj ?) Parona, Nuove osservaz. s. 1. fauna 



e I'eta degli strati con Posidonmnya alpina nei Sette Commimi 



Pal. Ital., vol. i, p. 10. 

 1897. Ph. sp. aff. tortisidcatum (d'Orbigny), Hochstetter, " Die Klippe 



St. Veit bei Wien," Jahrb. kk. R.A., vol. xlvii, pt. i, p. 142. 

 1897. Sowerbyceras Neumayri Parona & Bonarelli, " Sur la Faune Callov. 



Infer. Savoie " : Mem. Acad. Sc. Savoie, ser. 4, vol. vi, p. 120. 



Dimensions. — Diameter . . 15 mm. 



Height of the last 



whorl . . 53 per cent of the diameter. 



Thickness . .46 ,, ,, ,, 



Umbilicus . .18 ,, ;, ,, 



Descrijytion. — This specimen is characterized b}" a subrectangular 

 whorl-section with depressed venter, which is peculiar to Fontannes's 



