GEOLOGICAL AND MINER ALOGICAL NOTES, 



NO. 4. 



THE EXTENT AND PROBABLE THICKNESS OF THE CRYSTAL- 

 LINE CAMBRIAN DEPOSITS IN ESSEX COUNTY, MASSACHU- 

 SETTS. 



BY J. H. SEARS. 



I Supplementary to Notes No. 2, Bulletin Essex Institute, Vol. 22, 1890.] 



IN the paper printed in the twenty-second volume of 

 the Essex Institute Bulletin as Geological and Mineralog- 

 ical Notes No. 2, I gave an account, as far as was then 

 known of the extent of the Olenellus Cambrian rocks of 

 this region. While that paper was in press, however, 

 another deposit of this rock was located at Jeffry's Ledge, 

 about twenty miles east-northeast from Cape Ann, con- 

 taining numerous fossils of Hyolithes and Stenotheca, thus 

 uniting this last-named outcrop with the Olenellus Cam- 

 brian deposits of Nahant. Since then I have found several 

 other outcrops of these crystalline Cambrian sediments in 

 various parts of the county. One in Rowley, chiefly in 

 the valley between Hunsley and Bradford hills, but occa- 

 sionally rising to an elevation of one hundred feet, is 

 composed of a series of schistose argillite shales, ferru- 

 ginous sandstones, and cherty limestone which is much 

 metamorphosed in bands of light and dark color. Micro- 

 scopical examination shows this limestone to be composed 



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