,^ 



m 



218 Outlines of the Mineralogy and Geology 



Ihe peninsula which connects Nahant with the main, and pro- 

 ceeding to the Porphyry formation in Lynn, it runs at Its foot. 



r 



southwesterly to Maiden, und thence westerly, bounded by Pe- 

 trosilex, to Medford near the banks of Mystic river ; here it turns 

 northerly, and running in between Greenstone, it meets in Wil- 

 mington and Reading, tlje great alluvion, which comes in through 

 Chelmsford &c. from New Hampshire 5 thence, having a souths 

 west course generally, through part of AVest-Cambridge and 

 Walthara ; it meets the Greywacke in Newton, and bounded by 

 this formatioii, it runs southeast, through part of Brighton, Brook* % 



lin^v **^nd iioxbury to Dorchester^ where it meets the ocean, and 

 the southeast boundary is the coast, from this place to Nahant, 

 This deposition is interrupted in three places. At Maiden 

 and Charlestown hy Argillite ; and at Watertown by Argillite 

 and fine-grained Greywacke, which occurs here in small quantity; 

 these formations appear to be insulated in the alluvion. In the 

 peninsulas of Boston and Charlestown, and in some parts of Dor- 

 chester and Chelsea, are found the highest elevations of this dep- 

 osition, as for example, Bunker's and Breed's hill, Dorchester 

 heights, &c. and again, immense plains are formed hy it, as at 

 Cambridge and Waltham. Its greatest breadth is from four to 



five miles, and its narrowest portion a few rods only. At Sweet 

 Auburn, in Cambridge, this deposition appears to have suffered 

 some changes ; it is here formed into extensive ridges, of singular 

 regularity, and which are separated from each other by deep ravines; 

 they bear great resemblance to ancitnt fortifications. In other 

 parts, deep basons are shown, which contain small pools of water, 



