IGNEOUS BOOKS 473 



the granite on the west side of the fault. The quartz veins found in 

 other near-by localities, some of which attain a considerable size, can be 

 dated as contemporaneous with the silicification at Diamond Hill. 



Determinations of the silica percentages of the altered and unaltered 

 f el site show the extent of the replacement. The quartz rock on the west 

 face of the hill contains 98.6 per cent SiOg; the brown altered felsite 

 contains 95.7 per cent SiOg'; the felsite just east of the zone of maximum 

 silicification shows 76.5 per ceiit Si02, and the normal felsite 68.7 per 

 cent SiOo.^* This low silica percentage indicates that the rock is an 

 apodacite, the average SiOg per cent of dacite analyses being, according 

 to Daly,"*^"* 66.9 per cent. Together with the chemical evidence concern- 

 ing the nature of the rock is the petrographic evidence of the small 

 amount of original quartz and the large amount of plagioclase feldspar 

 and femic minerals, now highly altered. Therefore the rock may be 

 classed as an apodacite. 



Wood worth in his report on the Narragansett Basin briefly mentions 

 Diamond Hill, ascribing the origin of the quartz to the action of hot 

 springs following the decadence of igneous activity in the region. He 

 dates the quartz veins in Wamsutta time, but the lack of metamorphism 

 of the quartz veins indicates that they were not injected until toward the 

 close of the erogenic movements in Middle Pennsylvanian time. 



Two similar deposits of quartz have been described by G. F. Loughlin 

 in Bulletin 492 of the U. S. Geological Survey. The principal one is at 

 Lantern Hill, Ehode Island, and the other is at Swanton Hill, North 

 Stonington, Connecticut. The origin of the quartz in Lantern Hill is a 

 replacement of alaskite by pneumatolytic action. The quartz occurs in 

 distinct veins with well developed comb structure in a very similar man- 

 ner to that at Diamond Hill. The veins of the former deposit sometimes 

 contain a few irregular crystals of pink feldspar. The quartz is filled 

 with dirty specks and fluidal inclusions very much as that of Diamond 

 Hill. 



The Diamond Hill quartz has been quarried for a number of years. 

 'i1ie crushed and powdered quartz has been used for road metal, poultry 

 grit, roofing gravels, fireproof brick and fire sand. It is now used as 

 crushed stone for concrete and is being mined in a large cut in the west 

 side of the hill. 



Sheldonville quariz vein. — A short distance north of the village of 

 Sheldonville, Massachusetts, and 3 miles northeast of Diamond Hill is a 

 mineralized quartz vein which first attracted attention about 1896, and 



** Analyses by Barber and Mears in 1906. 



*^ Average chemical composition of igneous rock types. Proc. Am. Acad. Arts and Sci.. 

 vol. 45, No. 7, 1910. 



