PETROGRAPHICAL PROVINCE OF ESSEX COUNTY 29 1 



sion the reader is referred to "The Azoic System," by Whitney 

 and Wadsworth. 1 



These rhyolites are dense, black, aphanitic rocks, with a dull 

 or subvitreous luster and subconchoidal to even fracture. Small, 

 white feldspar phenocrysts are scattered through this black 

 groundmass. A banded or flow-structure is often noticeable, and 

 is especially well brought out on weathered surfaces. 



Under the microscope these rocks present a somewhat monot- 

 onous appearance. The feldspar phenocrysts are usually quite 

 sharply automorphic, less often fragmentary. Most of these are 

 of orthoclase, or rather soda-orthoclase, while a few show the 

 twinning lamellae and extinction angles of oligoclase-albite. 

 They are all somewhat decomposed so that optical examination 

 is unsatisfactory. 



The groundmass is composed of alkali-feldspar with some 

 finely granular quartz, very small shreds and grains of pale green- 

 ish pyroxene and a little magnetite. Glass is present in some 

 specimens, but in the majority of cases it has been devitrified, 

 and its former presence is difficult to determine with certainty. 

 Some of the specimens were apparently primarily noncrystalline. 

 Flow-structure is observed, but is not as marked as one would be 

 led to expect from some of the weathered specimens. These 

 rhyolites, it may be added, are accompanied by ash beds and 

 breccias. 



I owe to Mr. Sears a specimen of a dike rock much like these 

 rhyolites, which cuts the diorite on the south shore of Salem 

 harbor, west of Marblehead. It shows flesh-colored feldspar 

 and colorless quartz phenocrysts in an aphanitic groundmass. 

 In thin section it resembles the rhyolites, but is distinguished by 

 the abundance and sharp outlines of the quartz phenocrysts and 

 the presence of numerous spherulites in the devitrified ground- 

 mass, which exhibit a black cross between crossed nicols. 



x Bull. Mus. Comp. Zool., Vol. VII, Cambridge, 1884, pp. 331-565. Cf. also G. 

 H. Williams, Jour. Geol., Vol. II, p. 24, 1894. 



