CRYSTALLINE LIMESTONE 379 



Where it has been acted on by intrusive magmas the limestone has been 

 considerably metamorphosed, minerals of the amphibole and pyroxene 

 groups having been developed in large amounts. In some cases small 

 bodies of amphibolite and pyroxenite with gneissoid texture have formed, 

 the calcium carbonate having com]3letely disappeared (see figure 1). 

 Further details need not be given here, as typical occurrences of this for- 

 mation have been described in reports of the Pennsylvania Topographic 

 and Geologic Survey Commission.^ 



ORIGIN 



Assignment of a sedimentary origin to this limestone needs no justifi- 

 cation. 



Quartz-mica Schist 



character 



This formation is typically a pale greenish gray rock composed of 

 flattened lenses of quartz separated by films of sericite mica, through 

 which extend blades of sillimanite. A specimen is shown in figure 2. 

 This grades on the one hand into quartzite by the disappearance of the 



Figure 2. — Quartz-mica Schist with Sillimanite. (X %) 



Locality, 1 mile north of Springtown, Allentown quadrangle. Specimen 9243, The 

 sillimanite appears as white blades against the brownish gray quartz ; the upper part of 

 the specimen is stained red — which appears dark in the photograph — by iron oxide i-e- 

 sulting from the decomposition of pyrite. 



mica, and on the other into muscovite-schist as the quartz diminishes and 

 the mica becomes more coarsely crystalline. In addition to tlie sillimanite 

 several other minerals are occasionally present as accessory constituents, 

 and may locally become abundant; the most frequent are apatite, biotite, 

 garnet, ilmenite, pyrite, tourmaline, and zircon. 



8 Frederick B. Teck : Preliminary report on the talc and serpentine of Northampton 

 County. Topographic and Geological Surv. Penna., Rept. 5, 1911, pp. 12-23 ; Benj. L. 

 Miller : Graphite deposits of Pennsylvania. Ibid., Rept. 6, 1912, pp. 75-70. 



