G. H. Williams — Peridotites near Peekshill, JV. Y. 39 



member of the extremely interesting section exposed here for 

 350 feet and described by Professor Dana in his paper on the 

 Limestone belts of Westchester County.* The rocks here vary 

 considerably in their mineralogical composition at different 

 points. They are for the most part quite massive but not in- 

 frequently they exhibit signs of a schistose structure. This 

 Professor Dana was inclined to attribute to only partially ob- 

 literated stratification planes; but a microscopic study of the 

 rocks in question shows most conclusively that the structure is 

 one which was secondarily developed in massive rocks by the 

 action of great pressure. Scarcely any specimen is better cal- 

 culated to show the effects of such action on the minute inter- 

 nal structure than the above mentioned No. 54. The minerals 

 present are diallage, hypersthene, brown hornblende and a little 

 olivine. The principal constituent is diallage in large rounded 

 individuals which are frequently twinned and always possessed 

 of that peculiar, finely striated (almost fibrous) appearance, 

 which is well known to be the result of pressure.f These 

 diallage crystals are often bent and, no matter what may be 

 their crystallographic orientation, it can be seen that for all 

 the pressure acted from one constant direction. Around these 

 diallage individuals, and occasional hypersthene crystals which 

 have been subjected to like influences, extend finely granular, 

 curving bands of secondary pyroxene and brown hornblende. 

 This granular aggregate encloses the large, rounded and bent 

 pyroxene crystals like a groundmass and produces a structure 

 similar to that seen on a larger scale in the "augen-gneiss."^ 

 Such fine-grained aggregates of secondary minerals enclosing 

 the remains of larger original crystals have been admirably 

 described and illustrated by Professor J. Lehmann§ in the 

 " Augen-gneisses, granulites and " Flaser-gabbros " of Saxony 

 and by Professor K. A. Lossenf in the metamorphosed diabases 

 of the Harz Mountains. 



It is a fact not without significance in the case before us that 

 brown hornblende is a very abundant constituent of the second- 

 ary groundmass while it appears to be lacking among the origi- 

 nal constituents, although the larger individuals of diallage 

 appear in some cases to be passing into it by paramorphism. 



Two sections of Professor Dana's collection marked St. 3 (D) 



* This Journal, Sept., 1880, p. 218. 



f Vid. 0. Miigge, Neues Jahrbuch fur Mm., Geol. u. Pal., 1883, i, p. 84, and 

 van Werveke, ib., ii, p. 99. 



% For the German term for this structure : "flaserige or mikroflaserige St?-uktur" 

 there seems to be no exact English equivalent. It is common in the gneisses and 

 is sometimes called lenticular structure. The term "flaser-stnccture " might be 

 adopted for it from the German. 



§ Uhtersuchungen iiber die Entstehung der altkrystallinischen Schiefergesteine. 

 Bonn. 1884. 



|| Jahrbuch der kon. preuss. geologischen Landesanstalt fur 1883, p. 619. 



