DISLOCATIONS OF THE STRATA. 445 



to the main valleys of the Chilmark and Tisbury district; yet others may 

 be concealed beneath the covering of drift materials. In this connection it 

 is important to remark that the folds of the Gay Head series are not dis- 

 tinctly expressed in the topography, and but for the great section at Gay 

 Head there would be little opportunity to determine their existence. The 

 amount of dislocation in the Chilmark and Tisbury districts probably is 

 nearly if not quite as great as that at Gay Head. The average dip observed 

 at about a dozen points exceeds 45°, and at some points approaches the verti- 

 cal. The only place where a considerable section is revealed in a clear 

 manner, viz., at the east end of the Nashaquitsa cliffs, the amount of disturb- 

 ance is as great as in the most dislocated portion of the Gay Head section. 

 The total area of the dislocated rocks exhibited on Martha's Vineyard 

 exceeds 30 square miles. The most considerable width transverse to the 

 strike is three and one half miles. The degree of disturbance is about, in 

 a general way, equal in all parts of the section. The only field where the 

 rocks appear to be slightly dislocated lies immediately to the north of the 

 Chilmark pond and includes a surface not exceeding one-half a square mile 

 in area. In this portion of the field the beds, so far as determined by im- 

 perfect sections, maintain a nearly horizontal attitude. Taken alone, this 

 relatively undisturbed district might suggest a dying out of the orogenic 

 action in this part of the field, but considered in connection with the fact 

 that the section of Nashaquitsa cliffs indicates as intense disturbances as is 

 found anywhere else in the field, it seems more likely that this unaffected 

 area is a local accident. 



Age of the Martha's Vineyard Dislocations. 



The section at Gay Head is apparently divisible into two tolerably distinct 

 elements, viz: a lower division, the upper limits of which are not determined, 

 which is likely to prove of Cretaceous age ; and an upper part of the sec- 

 tion, which from the fact that it contains bones of cetaceans, is likely to 

 prove of Tertiary age — the two together forming the greater part of the 

 longitudinal section of Gay Head. Above these two more ancient portions 

 of the escarpment lie an extended series of unfossiliferous sands, which ap- 

 parently belong to a somewhat later age than the other portion of the section. 

 To this age we may also presumably assign the extensive series of beds 

 exhibited in the Weyquosque series. These later-formed beds are, at least 

 in the Weyquosque cliffs, deposited unconformably upon the earlier series. 

 A portion of these later unfossiliferous sands are involved in the contortions 

 at Gay Head, and a portion of them lie unconformably upon the edges of 

 the beds which were involved in the dislocation. It seems likely, therefore, 

 that this later series will in the end be found divisible into two parts — a 



