FOSSILIFEROUS MARINE CLAYS ' 529 



truncata, which was found abundantly in the formation. The overlying 

 sands were called "Saxicava sands" for the reason that they contain 

 Saxicava rugosa. Since that time the marine clays of the Saint Law- 

 rence valley and the New England coast have l^een studied by various 

 geologists, and the name "Leda clay" has been generally applied to any 

 marine clay in this region. It now seems clear, however, that the clays 

 classed under that name are not all of the same age. For this reason the 

 name "Leda clay" will be avoided, so far as possible, in this paper. 



General description. — Below the limit of su.rface oxidation the marine 

 clays are commonly blue drab to gray in color, finely laminated and 

 evenly bedded, often containing thin laminae of sand. There is consider- 

 able diversity of character; sometimes the clays grade horizontally into 

 fine sand, and in places they contain layers of "marsh mud" which has a 

 strong odor. Polished boulders are frequently found, and in places, as 

 in the Cambridge, Massachusetts, brick-yards, they are well striated. In 

 a number of localities they are abundantly scattered through the deposit, 

 indicating the existence of icebergs. The broad extent of the deposits 

 containing boulders on the New England coast, in all the large valleys, 

 and in the Saint Lawrence valley indicate, however, that the glaciers 

 from which the ice was derived may have been several hundred miles 

 distant. It is possible that at that time the highlands in the interior 

 of New-England and of the province of Quebec may have been ice- 

 covered and the coasts and the Saint Lawrence valley may have been 

 free from ice. Packard states that the greater abundance of arctic fos- 

 sils in the lower part of the clays and of forms characteristic of a milder 

 climate in the upper part indicate that the climate became milder during 

 the close of the clay deposition; hence it would appear fairly certain that 

 ■the main areas of clay containing marine fossils must have been deposited 

 during a retreat of the ice to a long distance. In thickness the clay is 

 irregular, ranging in Massachusetts, New Hampshire, and in parts of the 

 Kennebec valley in Maine up to over 100 feet, but it is commonly less 

 than 50 feet. 



Distribution.— The "Leda clays" are found far and wide throughout 

 the Saint Lawrence valley and on the coast of Maine, New Hampshire, 

 and in Massachusetts as far south as Boston. 



Clays which do not carry fossils, but which belong in this class, are 

 extensively developed in Caml)ridge, Medford, Everett, Eevere, and Chel- 

 sea, near Boston, and are found at Lynn, Salem, Saugus, Newburyport, 

 Ipswich, Amesbury, and Haverhill, in Massachusetts. At Lynn and 

 Danvers fossils have been found in them. In New Hampshire they are 

 finely developed at Exeter, Epping, Porismouth, Eollinsford, and Dover. 



