The Monoclinic Pyroxenes of New York State. 143 



is a direct change to compact blacli hornblende. Prof. Williams 

 has stated that the tendency of the pj^roxene molecule to assume 

 the form of hornblende is sufficient to affect a complete change in 

 the crystalline structure, but if " some external agency could be 

 introduced, which would render the molecules more or less mobile 

 without increasing the temperature to a point where the augitic 

 mode of arrangement is more stable than the hornblende, it will 

 be readily seen that the}^ must assume the form best in accord- 

 a,nce with the lower temperature." He also thinks that just such 

 an external agency as this would be furnished by the pressure to 

 which the rocks are subjected in mountain making (Ref. 59). 



A second and not uncommon alteration product is serpentine. 

 This change is well shown in the ophicalcites of Warren and Essex 

 Counties. The serpentine grains show a core of unaltered py- 

 roxene, and all stages in the transition may be seen. The com- 

 position of the serpentine and pj'roxene are given below (Ref. 38). 



Pyroxene. Serpentine. 



SiOa 55.26 42.17 



AI2O3 .22 .30 



FeA 22 1.57 



FeO 57 .64 



MgO 19.53 41.33 



QaO 24.48 



MnO tr tr 



H^O 13.72 



100.28 99.73 



As will be seen from the above analyses, the change consists in 

 the assumption of water and the loss of lime, which crystallizes 

 out as calcite. 



Chlorite has been observed as a result of the alteration of augite 

 in a few instances (Ref. 15, 48, 49). 



Pink garnet frequently appears to result from the alteration of 

 the augites in the Adirondack gabbros (Ref. 29). The two min- 

 erals occur in very close association, and the same cracks often 

 traverse both. The following is an analysis of the pyroxene and 

 the garnet : 



Pyroxene. Garnet. 



SiOa 56.00 43.38 



FeO 2.40 



CaO 21.63 22.61 



