176 The Monoclinic Pyroxenes of Neio York State. 



mixture, in microlites and large crystals, and associated with the 

 various minerals which accompany it in nature. Both tempera- 

 ture and chemical compositon exert an influence. Augite being 

 more stable at high temperatures will separate under such condi- 

 tions, whereas if the cr^'stallization takes place at a lower tem- 

 perature hornblende is more likely to result. According to Vogt 

 (Zeitschr, fur. Kryst. u. Min.), if the relation of CaO to MgO be- 

 comes as 1-3 orthorhombic pyroxene separates instead of the 

 monoclinic form. 



When an igneous rock intrudes itself into a limestone or dolo- 

 mite, a change usuall}' takes place in the latter along and near 

 the line of contact. This change consists first in an increase in 

 the coarseness of the limestone and secondly in the abundant 

 development of minerals in the contact zones or disseminated 

 through the limestone near it. This production of contact min- 

 erals has been frequently noticed and described. 



In contact metamorphism the intensity and extent of the 

 change depends on the temperature, mineralogical and structural 

 character of the eruptive rock, also on the duration of its action 

 and the area and conductivity of the rock affected. 



The contact changes are brought about partl}^ by the intrusion 

 of the igneous mass, and partly by the action of mineralizers. 

 The former can and have been produced artificially. 



The minerals of the contact zones derive their material 1. 

 from the igneous rocks, 2. from the limestone, 3. from below, 

 being brought up by solutions stimulated by the intrusion of the 

 igneous rock. The greatest development of contact minerals 

 occurs in an impure limestone, or if the eruptive rock mingles 

 with the limerock. (J. Roth p. 175.) 



"We should thus expect to find minerals rich in lime and mag- 

 nesia, such as garnet, vesuvianite, pyroxepe, wollastonite, amphi- 

 bole, chondrodite, etc. 



The presence of mineralizers is often shown by the occurrence 

 of fluorite, tourmaline and chondrodite, and indeed it has been 

 found that the presence of mineralizers is an essential factor in 

 aiding the formation of some of these minerals. 



Diopsides and augites poor in alumina are especiaylly aburldant 

 in the limestone contact zone, and their frequent occurrence at 

 such localities lining the sides of fissures filled with quartz or cal- 

 cite, rather point to a possible formation sometimes by wet 



