Granites of Cecil County, in North eastern Maryland. 105 



We may represent this change structurally in the following 

 manner : 



( (Si0 4 )=(A10) 4 ) f (Si0 4 )=Al 



\ / / \ -(Fe Al 3 5 )= / 



\ Al = (Si 4 )— (Fe OH) J +K ( Al=(Si 4 ) = (KH) 



which, while of course hypothetical, does exhibit the relaticn 

 between the staurolite and its observed alteration products. 



A number of other accessory minerals occur in this schist, 

 as garnet, magnetite, tourmaline, apatite, rutile and chlorite. 



Red garnets are seen 

 both in the schist and the 

 staurolite. They are filled 

 with small quartz inclu- 

 sions, and usually show a 

 more or less complete alter- 

 ation to chlorite. In some 

 of the crystals there is a 

 narrow peripheral rim of 

 chlorite, which increases in 

 size, at the expense of the 

 garnet, until only a small 

 core of the original min- 

 eral remains. The chlorite 

 forms such a perfect pseu- 

 domorph that the original 

 dodecahedral outline of the 

 crystal is preserved (vid Fig. 5). The alteration of garnet to 

 chlorite has frequently been referred to in the literature, and 

 good references to such a change may be found in the works 

 of Roth* and Rosenbusch.t 



Magnetite, as proven by the magnet, occurs in minute dots 

 or grains, and often with crystal form in both the schist and 

 the staurolite. It seems to be more abundant in the latter 

 mineral, and it is often grouped together so as to form 

 skeleton crystals. 



Tourmaline is present in the schist and staurolite in small 

 pleochroic, hemimorphic crystals, but it is never in large 

 amount. 



*Allgemeine und Chemische Geologie, Vol. I, pp. 35:>-:k).*i. 

 fMikroskop Physiographie der Mineralien, Vol. I, p. 447, 1S!'J. 



