REPORT ON THE GEOLOGY OF ESSEX CO., MASS. 129 



No. 17. Zoicite-Gneiss. 



This is still another member of the Cambrian Sediments 

 and should in places be called zoicite-epidote-gneiss. It 

 belongs to the series of metamorphosed slates and sand- 

 stones. 



No. 18. Liniestone, Slate, Quartzite and Sandstone. 



This formation comprises the interstratified members of 

 the Olenellus Lower Cambrian fossiliferous beds. 



No. 19. Conglomerate-Granite. (Arkose.) 



This is composed of large pebbles of granite, limestone 

 and mica-schist. 



No. 20. Bostonite or Keratophyre. 



Bostonite is the name given by Professor Rosenbusch to 

 a series of dyke rocks of the same composition as the kera- 

 tophyre, which is distinctly a surface flowing lava and not 

 a dyke. Careful investigation has proved that it Covers 

 a coarse breccia and other members of the rhyolite and 

 quartz-porphyries on a nearly level floor gently sloping 

 into Marblehead harbor. 



No. 21. Tinguaite Dyke. 



This dyke is seen in Manchester cutting the hornblende 

 granitite and augite-nepheline-syenite at Pickard's point. 

 It is the only recorded occurrence of this rock formation 

 in Massachusetts. 



HYPIDIOMORPHIC STRUCTURE. 



No. 22. Essexite. 



This formation is found in numerous outcrops on Salem 

 neck, Winter island, and at Beverly and Marblehead. It 

 is the type of a basic-augite-nepheline rock, quite por- 

 phyritic, and of a nearly black color. It is very different 



ESSEX INST. BULLETIN, VOL. XXVI. 17 



