462 WARREN AND POWERS DIAMOND HILL-CUMBERLAND DISTRICT 



Northeast of Grants Mills is developed what appears to be a flow of 

 felsite which has incorporated a great number of angular fragments of 

 felsite — a felsite flow breccia. The fragments vary in size, one-quarter 

 of an inch being perhaps an average size. The rock is badly weathered, 

 films of limonite being developed about the felsite fragments, so that they 

 can be easily . separated from the matrix. Farther north is a sheared 

 phase of the same rock. 



It appears that these felsites represent a series of flows from a fissure 

 or fissures or a central vent probably located under Diamond Hill. The 

 flows are of slightly varying composition (rhyolitic or dacitic), as indi- 

 L-ated by their microscopic characters, and they were in all probability ex- 

 truded in Wamsutta (Lower Pennsylvanian) time and presumably had 

 a much wider extension formerly than at present. The contact between 

 the Xarragansett series 'is not exposed and it is therefore not certain 

 whether the flows are interbedded with this series or not. Felsite does 

 occur interbedded with the sediments at a point several miles away (see 

 below). Neither can the direction of the flows be very well determined. 



Wamsutta volcanoes. — ^Yoodworth found a series of interbedded dia- 

 bases and felsites in the Wamsutta formation of the Narragansett series. 

 These outcrop around the margin of the horseshoe-shaped fold in which 

 the Wamsutta is exposed. The felsites are found near South Attleboro, 

 the diabases from Xorth Attleboro to South Attleboro, and around the 

 horseshoe up toward Arnold Mills; also there are some amygdaloids as 

 exposed near 158 Elm Street, North Attleboro. Later studies have 

 shown that these igneous rocks are probably surficial lava flows. Wood- 

 worth says : 



"The peculiar features of the Wamsutta series — the rapid thiclveniiig of the 

 sandstones and conglomerates toward the northwest corner of the present 

 area, the felsites with definite flow structure, the gray ash heds or Attleboro 

 sandstone, the agglomerates composed in large part of felsite pebbles — all point 

 to a volcano or volcanoes existing in this field in Carboniferous time." *^ 



Woodworth concluded tbat the source of these volcanics was probably 

 from vents now occupied by the granite porphyries in Cumberland. It 

 seems more probable, however, that this series of lava flows represent 

 local fissure eruptions. Diamond Hill is composed of felsite of similar 

 age to that at South Attleboro, but the distance between these two places 

 is over 6 miles. Furthermore, the sti'atigraphic distance is 7 or 8 miles 

 i)ecause of the folding of the Narragansctt series. Therefore it seems 

 best to explain the South Attleboro felsites and diabases and the Diamond 

 Hill felsites by local fissure eruptions of contemporaneous age. 



" Monograph TT. S. Geol. Survey 33, p. 1.55. 



