Granites of Cecil County, in North-eastern Marytand. 83 



The epidote is often arranged in zones, and in very many 

 cases a large amount of the mineral is concentrated in a rim 

 within the feldspar slightly removed from the edge, as is 

 shown in Fig. 2. The interior of such feldspar crystals con- 

 tains less epidote, and shows clearly the twinning lamellae. 

 More rarely the epidote concentration occurs at or near the 

 center of the feldspar. 



Since the zonal arrangement of the epidote occurs in 

 plagioclase feldspars composed of a mixture of the albite and 

 anorthite molecules, its ex- 

 planation must be sought 

 in variations of chemical 

 composition within con- 

 centric zones of a single 

 felspar crystal. Such varia- 

 tions have been proven to 

 exist and have been care- 

 fully described by Hopf- 

 ner* and Becke,t who noted 

 the fact that, while in the 

 main, zonal feldspars grow 

 more and more acid from 

 their centers outward, there 

 is, nevertheless, a frequent Fm. 

 recurrence of zones more 

 basic than some of those 

 within them. Hence there is often more or less irregularity 

 in the succession or arrangement of zones especially rich in 

 the lime molecule, which is probably caused by oscillations 

 of the physical conditions within the magma previous to 

 its solidification. 



Exactly what the chemical and physical conditions were 

 which brought about so extensive a development of epidote 

 within the feldspar of the Rowlandville granite, it is quite 

 impossible to say. That it has been in some way connected 

 with the action of the orographic pressure to which these 

 rocks were subjected, appears altogether probable, especially 



!. — Plagioclase feldspar, twinned, and 

 showing rim of epidote near the 

 border of crystal . x 80. 



: " Ueber das Gestein des Monte Tajnmbina," Nenes Jahrbuch, p. 182, 1881 (2J. 

 t '' Petrographische Studien am Tonalit der Rieseferner," Tschermak Min. 

 Mitth XIII, p. 411, L893. 



