178 The 3Ionoclinic Pyroxenes of New York State. 



phosed magnesian limestones of West Chester, Dutchess and 

 Putnam Counties has been described and it was mentioned that 

 the pja-oxenes occurred in bunches filling cavities or in streaks 

 associated with quartz. So far as the writer is aware, none of 

 the occurrences in these localities are associated with igneous 

 intrusions, and the probability, therefore, is that these p5^roxenes 

 were produced by the regional metamorphism to which these 

 dolomites were subjected. Two methods of formation are, 

 therefore, possible : 1. Segregation from the surrounding rock ; 

 2. Deposition from solutions which brought the material from a 

 deep-seated source. The presence of associated quartz and the 

 occurrence of the crystals in veins or cavities would seem per- 

 haps to favor the second hypothesis and still it is possible that 

 the pyroxene in these dolomites has been caused by a segrega- 

 tion of material from the surrounding limestone which contains 

 all the elements needed in the formation of these leucaugites. 



The formation of p3a'oxene by regional metamorphism has been 

 described by Westgate from Warren Co., N. J. (Amer. GeoL, 

 1894). W. H. Hobbs has also described the formation of tremo- 

 lite and large pyroxene crystals along the Housatonic fault, in 

 Massachusetts (The Structure of the Housatonic Valley lying 

 east of Mt. Washington, Jour. Greol.), and Fairbanks mentions a 

 similar case of the formation of green pyroxene in California 

 (Geol. of the Coast Ranges of Cal., B. G. S. A., YI, p. U). 



In the last class of occurrences mentioned, viz., the formation 

 of pyroxene around ore bodies, there is still considerable doubt 

 as to the theory of the origin of most of the pyroxenes. They 

 may have been formed by segregation, but this is a point for fur- 

 ther investigation. 



