255 



The Islands North of Hinghmn. 



Slate Island and Grape Island, although belonging politically 

 the one to Hull and the other to Weymouth, are geologically 

 a part of Hingham ; and at extreme low tide they are 

 topographically almost joined to the main land. These two 

 islands are contrasted in their topographic features. Slate 

 Island, the name of which is unusually appropriate, is almost 

 bare of drift ; and with the exception of some unimportant 

 beach deposits on its southern shore, the ledges of slate are 

 exposed over, virtually, its entii'e surface, or would be but for 

 the exceedingly dense growth of sumach, ras})berry, and other 

 shrubs. The island rises abruptly on the north shore in a low 

 cliff from 15 to 25 feet high, and descends thence in one long- 

 gentle slope to the low southern shore. Grape Island, on the 

 other hand, although clearly underlain by slate, is heavily 

 drift-clad, bearing two distinct drumlins, with a broad depres- 

 sion holding a small pond between them. 



Slate Island, especially, is, geologically, essentially a 

 continuation of the northwest shore of Hingham. With the 

 exception of the numerous rusty gray dikes of diabase, the rock 

 is ail the same soft, dark gray, thin-bedded slate with wliich we 

 have become familiar north and northeast of Huit's Cove, 

 the chief difference being that on the island the slate is less 

 contorted, and has a higher and more constant dip ; and the 

 cleavage and bedding are in constant instead of only occasional 

 ajjreement. The strike is usuallv N. 65°— 70° E., the exti-eme 

 range, however, being N. 55°-75°. The dip is almost 

 vertical over the wliole surface of the island, varying usually 

 between S. 85° and 90°, although at one point, where 

 influenced by a large dike, it is N. 85°. The stratification is 

 everywhere very thin, even, and regular; and many of the 

 exposures are exceedingly beautiful examples of the upturned 

 leaves of the geological record. Smooth, glaciated surfaces 

 present a remarkably perfect striping or lining ; while on the 



