POST-GLACIAL HISTORY OF BOSTON. 445 



b. Exeter Street. — The surface of the street here is 10.5 feet above 

 mean low tide, while the bottom of tlie peat bed is 15.5 feet below 

 mean low tide. 



Feet 



1. Fill, in blotting out the Back Bay 16 



This fill has taken place mostly since 1868. 



2. Gravelly black silt. Few fossils 6 



3. Fine black silt. ]\Iany fossils 5 



The middle portion is very full of fossils; the uppermost two 

 feet and the lowest foot contain but few. This silt is a dark- 

 grey (when dry) argillaceous sand with a considerable number 

 of mica scales. The compoimd microscope shows that very 

 minute sand particles made up fully nine-tenths of the mass; 

 there is merely sufficient clay and carbon particles to give 

 consistency and a dark-grey color to the sediment. 



4. Sandy, fresh-water peat 5 



5. Blue sandy clay with some peat 1 



This is the upper edge of the glacial deposit. 



c. Copley Plaza Hotel, on Copley Square at corner of Dartmouth 

 Street and Huntington Avenue. During the excavations for the 

 foundations Mr. C. W. Johnson obtained, at a depth of 25 to 30 feet 

 below the street surface, Mya arenaria and Macoma balthica, both 

 similar to the Exeter Street forms. The silt in which they occurred 

 was similar in color and composition to that of Exeter Street,: — 4b3. 



At Dartmouth Street in the Subway the upper ten feet of blue clay 

 (beneath the black silt) was so hard that a pickax could penetrate it 

 only with difficulty; below that it gradually became quite soft. This 

 hardening was probably due to the oxidation of the surface and the 

 deposition of iron as cement upon exposure to air. The presence of 

 ancient gullies in the surface of the clay encountered at various places 

 in the subway also points to this subaerial exposure. 



d. Clarendon Street. At a depth of 25 feet below the surface 

 of the street were found, — 



Mulinia lateralis c 

 Macoma balthica c 

 Gemma gemma c 

 Ilyanassa obsoleta c 



These forms are all like those from the Exeter Street locality and 

 occur in a similar silt. 



e. Berkeley Street. Surface of street is about 16 feet above low 

 tide. 



