SSO Outlines of the Mineralogif and Geology 



Greenstone. 



(Coloured on the map, green.) 



This is bounded by an alluvion and Sienite, nortli ; east, by 

 Porphyry; by the Cambridge alluvion, south, and extends north- 

 erly and westerly beyond the compass of these observations. 

 From the Petrosilex^ in Maiden and Lynn, it runs westerly, and 

 spreads through Medford, Stoneham and Reading, to the alluvi- 

 al strip, <vhich connects the alluvion at Cambridge, and with 

 that at Wilrainirton ; on the west side of this strip, it includes 



the town of Woburu, Lexington, Lincoln, Weston and part of 

 West-Cambridge and Waltham. In this extent, are several 

 ponds of fresh water, of considerable magnitude. 



A Greenstone formation prevails from Lynn to Marblchead, 

 and Salem. It is separated from Sienite, on the west, by an al- 

 luvial deposit; it is intimately associated with Sienite, and no 

 Letter line of deraarkation between them, than this alluvial strip, 

 is presented. 



GreywacJce.y 



(Coloured on the map, grey.) 



This is an oblong strip, about two miles wide and eight or frn 

 long. Its direction is southeast and northwest. The southeastern 

 extremity is bounded by the waters in Boston harbour and by an 

 alluvion in Dorchester. In Milton, it forms the falls of Nepon- 

 set river, in the centre of the town, thence passing through Dor- 

 chester, Roxbury, Brookline, Brighton and Newton to Needliam, 

 where it is bounded on the southwest, by an alluvion and Pe- 



trosilex. It forms Squantum, part of Hingham, and 



app 



3 



byP 



Nantasket beach. In Hingham it is bounded by Sienite 

 tsikx and the oceau. Its borders in manv places pass int< 



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