CAPE COD GEOLOGY 



313 



GLACIAL ERRATICS ON NAUSHON 



Some of the numerous glacial boulders on Naushon are grouped in huddles 

 or lines. In places along the shore where the finer drift has been washed away 

 boulders have been heaped up in irregular piles by the waves, as for example, 

 about a mile northeast of Tarpaulin Cove (Plate 38, fig. 2). Large flattish 

 boulders that are piled up along the coast have a dip seaward that shows the 

 action of storm waves. Boulders 6 feet long have been thus thrown up by the 

 storm waves of Vineyard Sound. Boulders of conglomerate derived from Car- 

 boniferous rocks around Middleboro are rare on the island. Most of the 

 boulders appear to have been brought from the pre-Carboniferous gneiss, schist, 

 and diorite about New Bedford and beneath the waters of Buzzards Bay. 



Among the rocks seen were a coarse-grained schistose diorite ; an aplite dike 

 rock in contact with blackish schistose hornblende granitite; a black actinolite 

 rock ; a gray granite-gneiss carrying large inclusions of a subrounded fine-grained 

 dark rock, possibly diorite; a large boulder of coarse-grained pepper-and-salt 

 diorite containing veins of epidote and coarse dioritic segregations; a fine-grained 

 diorite containing intrusions of granitic rock; a diorite breccia enclosed in 

 granite like that at Little Nahant; a mass of fine-grained aplite (?) enclosing 

 angular blocks of a coarse, gray hornblendic granitite; a brownish, massive 

 semi-schistose rock containing aggregates of hornblende in drawn-out bunches, 

 and a gneiss containing well defined crystals of feldspar that weather out and 

 leave pits 1| inches across. One hornblende-granitite block exhibited a dike of 

 fine-grained basic rock, which showed a gneissic structure formed by shearing 

 along a fault zone. One cobble of diabase was seen. 



A comparison of these rocks with those found north of Boston, near 

 Worcester, and around the head of Narragansett Bay indicates that the area 

 around New Bedford and probably the islands are underlain by a group of 

 chiefly igneous rocks that were intruded in probably the following order: 



1. Dioritic schist (possibly a metamorphosed phase of No. 3). 



2. Granite-gneiss carrying shreds and blocks of No. 1. 



3. Diorite of second (?) intrusion. 



4. Hornblendic granitite with aplitic and pegmatitic dikelets, some enclosing 

 brecciated facies of the mother rock. 



5. Aplite dikes of No. 4, contemporaneous or later. 



The schistose rocks in the area around New Bedford, which were separated 

 from the Carboniferous rocks and the granites by Edward Hitchcock in 1841 



