EVOLUTION OF LIMESTONE AND DOLOMITE 



397 



instability of calcium at contacts as compared with magnesium 

 is suggested by the tendency toward increase in the magnesium 

 content of altered sediments, as compared with their unaltered 

 equivalents. See Tables X, XI, XII. The averages of the 

 tables are misleading since they do not represent the tendency of 

 the majority of cases. 



TABLE X 



Contact Metamorphism of Slates by Acid Intrusives 



Rock 



Chloritic phyllite 

 Slate 



Slate 



Slate 



Slate 



Yaqui Gulch . . . . 



Morenci shales. . 



Composite 



Average 



Altered by granite (Neues Jahrb., 1897, 



P- 156) 

 Altered by hornblende granite 50' from 



contact (Hawes, Am. Jour. Sc, Mt. 



Willard, N.H.) 

 Ditto, 15' from the contact 

 Ditto, 1' from the contact 

 Ditto, at contact 

 Contact with quartz diorite (Bull. 150, 



U.S.G.S.) 

 Contact with porphyry (Bull. 22 g, 



U.S.G.S., p. 348) 

 Of 6 slates by acid intrusives 



TABLE XI 



Contact Metamorphism of Slates by Basic Intrusives 



Rock 



Composite. . . 



Lenneschiefer. 



Lenneschiefer. 

 Slates 



Slates 



Virginia 



Carboniferous 



Average. . . 



8 adinoles altered by diabase (Roth, 

 Geo/.,* Ill) 



Composite of 3 slates altered by dia- 

 base (ibid.) 



Ibid., 147 



Composite of 3 slates altered by dolerite 

 (Crystal Falls, Mon. 36, U.S.G.S.) 



Clausthal altered by diabase dikes 

 (Groddeck, Jahrb. der koniglich- 

 preuss. Landesanstalt, 1855, pp. 1-53) 



By gabbro intrusive (Mon. 43, U.S.G.S., 

 170) 



Shale by peridotite dike (Bull. 348, 

 U.S.G.S., 343) 



