THE STRATIFIED ROCKS OF ESSEX COUNTY. 33 



which have been identified by Mr. Charles D. Walcott of 

 the United States Geological Survey, Washington, D. C. 



The strike of this limestone is 18 north of east, the dip 

 40-43 west. 



In the month of July, 1890, I discovered an outcrop of 

 this Olenellus limestone in a valley between Prospect Hill 

 and Hunslow's Hill in Rowley. It has nearly all become 

 altered to chert and epidote, but fragments of the Hyo- 

 lithes fossils are still to be found in it. 



This outcrop dips under a red sandstone which in turn 

 is covered with the fine grained granitic gneiss of the re- 

 gion. The strike of this newly discovered outcrop cor- 

 responds very nearly with that of the Nahant rock of 

 similar character, and is 20 north of east with a dip 45 

 west. A mass of diorite, known as Metcalf's rock, cuts 

 across this limestone on the southeast near the Ipswich 

 line and on the north it is covered by the banded red fel- 

 sites of By field. 



Near Bennett's Head, Nahant, the strike of this lime- 

 stone is 20 west of north, dip 45 southeast. Here the 

 limestone rock mass has been turned or pushed one side by 

 the intrusion of a massive dyke of very unusual character, 

 and which under the microscope in thin section is seen to 

 be composed of hypersthene, olivine somewhat serpen- 

 tinized, diallage, plagioclase, biotite, numerous brown zir- 

 cons, magnetite, a little calcite and brown hornblende. 



At a short distance north of Bennett's Head, Nahant, 

 there is exposed at low tide a metamorphic schist having a 

 strike northeast to southwest. It is again seen at Bass 

 Point in the southwest part of the town. The microscopic 

 structure of this schist in thin section is : grains of quartz, 

 some feldspar in bands alternating with dark bands com- 

 posed of grains of quartz, grains of magnetite in large 

 amount, flakes of biotite, some flakes of chlorite, muscovite 



ESSEX INST. BULLETIN, VOL. XXII 3 



