116 F. BASCOM — VOLCANICS OF NEPONSET VALLEY 



dij)s to the north under a slate, and along tliQ southern margin the dips 

 arc to the south, and the conglomerate passes again under the slate. 

 This structure is complicated by several sharp Bynclines and numerous 

 faults. 



In the western part of the valley erosion has uncovered the volcanics. 

 The acid volcanics are the floor on which the conglomerate rests, while 

 the basic volcanics occur as dikes and as flows interbedded with the 

 conglomerate. There are three flows of non-porphyritic basic lava and 

 one of porphyry. 



Age of the Rocks 



The age of the conglomerate is Carboniferous. 



The acid volcanics must be pre-Carboniferous, while the basic igneous 

 material is of the same age as the conglomerate. 



The rocks come to the surface in three considerable areas with as many 

 small outlying bodies. The larger areas are severally 1, 2, and 5 miles 

 approximately in length and possess a mean width of one-half mile. 



Acid Volcanics 

 macroscopic characters and occurences 



The acid volcanics are more extended in areal exposure than the 

 basic volcanics. They are both tuffaceous and massive. The fragmental 

 material is readily recognized by mottled weathered surfaces, which owe 

 their character to variousl} 7, colored fragments contained in a light green 

 or pink base. A fluxion arrangement of the fragments is sometimes 

 marked. 



The massive effusives exhibit a considerable range of colors and 

 textures. Light green, gray, various shades of pink and purple, and a 

 brilliant brick red are the notable colors. They frequently possess a 

 compact cryptocrystalline felsitic texture, and are banded with light and 

 dark red tints which reveal conspicuously curving and crumpled lines 

 of flow movement. This fluxion banding is accompanied by an easy 

 cleavage of the rock into slabs parallel to the fluxion planes, precisely 

 as has been noted in the case of similar acid volcanics from the Lipari 

 islands. This t} r pe is also very brittle, breaking with sharp edges which 

 cut like glass. 



The rock is usually non-porphyritic in the hand specimen, though in 

 some instances it shows small and inconspicuous phenocrysts. It is 

 somewhat rarely amygdaloidal, when the amygdules are of red jasper, 

 and are characterized in a marked degree by the lenticular shape and 



