C. H. Smyth, Jr. — Peridotite in Central New York. 325 



still another constituent of the ground mass, of imperfect lath 

 shape, and in color white, green or brown. There is a pro- 

 nounced fibrous structure, perpendicular to the long axis, pro- . 

 ducirig an appearance very similar to that resulting from the 

 serpentinization of olivine along cracks. Both single and 

 double refraction are high and extinction is parallel. The long 

 axis is sometimes that of greatest, and sometimes of least 

 elasticity in the section, showing the plane of the optic axes 

 to be perpendicular to the long axis. This is also proven by 

 the behavior of the mineral in convergent light. The mineral 

 is decomposed by hydrochloric acid giving a jelly easily stained 

 by fuchsine, and its color changed to red by ignition. All of 

 these characteristics indicate that the mineral is olivine, in par- 

 tially developed crystals. But in view of the rarity of a second 

 generation of olivine in porphyritic rocks, it is possible that 

 further study of better material may modify this conclusion. 



The highly basic character of the rock indicated by its 

 mineralogical composition, is clearly shown by a partial analysis 

 given below. For comparison, the figures for the same con- 

 stituents are reproduced from published analyses of other rocks 

 of like nature. 



I (Smyth). II (Morrison). Ill (Peter and IV (Hunt). 



Kastle). 



Loss on ign 15'20 12-67 16-55 12-77 



Si0 2 33-80 37-44 29'43 40-67 



A1 2 3 6-84 28-60 2-36 5'13 



Fe 2 3 12-26 11-92 9-06(FeO) 8-12(FeO) 



CaO 9-50 5-45 6"94 



MgO 21-38 1-97 31-66 32*61 



Sp.gr. ... 2-732 2-697 



I. Manheirn Dike. 

 II. Ithaca Peridotite.* 



III. Elliot Co., Ky., Peridotite.f 



IV. Serpentine from Syracuse Peridotite. \ 



The lack of closer agreement in composition of these four 

 nearly related rocks, is readily explained by the strong tendency 

 to decompose manifested in all members of this very basic 

 group. But despite decided variations, they are all very low 

 in silica and, with the exception of ISTo. II, high in magnesia 

 and iron. The altered condition of the Manheim rock is shown 

 by the loss of 152 per cent on ignition, and by its ready ef- 

 fervescence with acids. Though the iron was determined as 

 Fe 2 3 , it doubtless exists in the rock as FeO and Fe 3 4 . The 



* This Journal, vol. xlii, p. 412. f Bulletin 38, U. S. G. S., p. 24. 



X This Journal, vol. xxxiv, p. 138. 



