210 TRANSACTIONS OF THE [1892 



general it is seen in exposures rising only from three to 

 five feet above average tide. But wherever it has been 

 disturbed by erosive agencies, its surface is broken, and 

 changed to the many colored variegated clay used by the 

 Indians to construct their pottery. 



As no fossils have yet been identified from this deposit 

 of clay, it is not possible to correlate it precisely with that 

 of the Amboy and Woodbridge type, or with the true 

 " Yariegated Clay " of the summit of the Potomac forma- 

 tion. It is massive, and shows but few indications of 

 stratification ; and none of its exposures yielded to me 

 either fossil leaves or seeds. In texture it is much like a 

 lead-colored clay in the Woodbridge region ; but it also 

 resembles the Potomac clay in the upper part of Federal 

 Hill, in Baltimore. 



Whatever may be the exact position of this clay in 

 relation to the Potomac formation, it occupies the horizon 

 of the lower Cretaceous deposits, and by the now known 

 position of the two series which overlie it, no correspond- 

 ence can be admitted between it and the Eocene parti- 

 colored clays of the Isle of Wight. 



As we have remarked above, this once thick sheet of 

 plastic clay has been profoundly eroded on its eastern side, 

 but small areas on that side, as at Nashaquitsa and vicinity, 

 have been left as monumental remains, to be still further 

 degraded by the sea and rain. Multitudes of small 

 exposures of the pale upper part of this clay occur at inter- 

 vals all the way from Gay Head to near Cottage City. 

 The whole island of Martha's Vineyard is no doubt under- 

 laid by this deposit. B}' reason of its type of denudation, 

 it forms a sinuous axis for all the succeeding beds on the 

 western half of the island. While on the eastern side it 

 supports chiefly the glacial and modern deposits on its base- 

 levelled surface. 



The structure of the Gay Head terrane admits of no 



